Medical records indicate that John Straffen, the notorious child murderer and Britain's longest serving prisoner, should never have stood trial in 1952.

Straffen, now 71, who on August 9 will have been in prison for exactly 50 years, had been found unfit to plead at an earlier hearing. His lawyers are now applying to the criminal cases review commission to have his case reopened on the grounds that the judicial process was not valid, and that, even if it was, Straffen's subsequent conviction is unsafe.

Having confessed to the murders of two young girls in 1951, Straffen was sent to Broadmoor, and subsequently sentenced to death the following year for the murder of a third girl - a crime he has always denied. He was later reprieved.

Straffen was originally arrested for the murder of nine-year-old Cicely Batstone on August 9 1951. He quickly confessed to the murder, and to the earlier one of six-year-old Brenda Goddard, and was sent for trial at Somerset assizes in Taunton in October that year.

The judge, Mr Justice Oliver, told the jury: "In this country we do not try people who are insane. You might as well try a baby in arms". Straffen was found unfit to plead and no trial was held. He was committed to Broadmoor asylum for the criminally insane, to be detained at His Majesty's pleasure.

On April 29 1952, Straffen escaped for almost four hours. The following morning, the body of five-year-old Linda Bowyer was found. Straffen was charged with her murder. Three doctors asserted that he was now fit to plead, partly on the basis that he understood four of the 10 commandments. The case was heard at Winchester and Straffen pleaded not guilty.

The trial was abandoned after a juror was heard to say that he thought Straffen was innocent and the crime had been committed by one of the witnesses. Amid great publicity, the judge, Mr Justice Cassels, stopped the trial, empanelled a fresh jury and the proceedings began again.

Straffen was then found guilty and sentenced to hang. After his appeal was turned down, his defence counsel, Henry Elam, said the issues raised were of such profound public importance that he would appeal to the House of Lords. However, the application for leave was refused.

The execution was fixed for September 9. However, against a background of immense public concern - according to the journal, Medical World, it was not the sanity of John Straffen that was in question but the sanity of the criminal justice process - the home secretary, Sir David Maxwell-Fyfe, reprieved Straffen. The reason for the reprieve was never made public.

Straffen's lawyers have finally gathered together much of the documentation in the case. The newly-released medical records make it clear that, as has long been suspected, Straffen was reprieved on the grounds of insanity.

According to contemporary NHS records, "If the doctors, or the majority of them, certify that the prisoner is 'insane', sentence of death is never carried out but the prisoner is reprieved and detained indefinitely, usually as a Broadmoor patient.

"It was by virtue of this procedure that Straffen, a mental defective, was finally reprieved," the records said.

Exceptionally, he was not sent back to Broadmoor, but instead has spent all 50 years in maximum security prisons throughout Britain. Straffen is currently in a prison in central England and still insists he did not kill Linda Bowyer.

For the majority of his time in prison Straffen has had no legal representation but soon after the CCRC was set up in 1997, he applied on his own behalf to have his case reopened. The CCRC was inclined to turn down the submission but the case has been taken up by Hadgkiss, Hughes and Beale, a Birmingham-based firm of solicitors.

The CCRC has now agreed to hear renewed representations on Straffen's behalf.

Should it be referred to appeal, he would receive legal aid.

John Straffen was 21 and had already spent half his life in various institutions when he was first arrested for murder on August 9, 1951. With the exception of four hours of freedom after escaping from Broadmoor, he has been in prison ever since. Now in his 50th year of imprisonment, he is easily Britain's longest-serving prisoner.

The world Straffen left behind in 1951 was, it goes without saying, very different. Clement Attlee was prime minister and George VI was king. The middle-market newspapers that avidly reported the crime he was accused of cost three-halfpence; and the newly appointed chairman of the BBC governors, Sir Alexander Cadogan, revealed that he did not possess a TV set and, indeed, had never seen a British television programme. In many respects, it was an age of tranquillity, when mothers were unconcerned when their young daughters ran off on their own to pick wild flowers, or even went unaccompanied to the cinema.

On Sunday, July 15, Straffen came across six-year-old Brenda Goddard on Rough Hill, at the back of her home in Bath. "She was picking flowers," Straffen would later say, "and I told her there was plenty higher up." She walked with him to a nearby wood, where he put his hands around her neck and strangled her to death. According to Broadmoor records, he said he did it to give the police "something really to do" instead of doing him for trivial offences. Brenda had gone out shortly before 2pm, and her guardian, Doris Pullen, missed her almost straight away, as she needed to get her ready for Sunday school. She and her husband, Arthur, searched for her, before going to the police at 3.15pm. The body was discovered at 7.10pm that evening. Mrs Pullen described seeing a man aged "about 35" walking from near where the body was found. Bath police worked their way through the local suspects, and on July 31 interviewed Straffen. Though he did not mention the murder, he brazenly placed himself in the area, even saying that he'd walked into the wood where the murder took place. "I read the description of the man wanted for interview in connection with the murder," he added, "but as I am only 21, I did not think it referred to me." At that stage, the police had no evidence, and Straffen was not arrested. However, his boss found out that he'd been interviewed in connection with the inquiry, and sacked him. Straffen now had another reason to want to provoke the police.

A few days later, having by then made a second statement in connection with the Goddard murder, Straffen went to the cinema. He loved going to the pictures, and that week the Forum was showing Tarzan And The Jungle Queen. Straffen started talking to nine-year-old Cicely Batstone. "It was very rare she went to the pictures on her own," explained her mother, "but it was rather a children's picture, so I let her go."

Afterwards, Straffen persuaded Cicely to go with him to the Scala to see another film, She Wore A Yellow Ribbon. This involved catching a bus across town. When they left the cinema, Straffen took her to a field known locally as the Tumps and strangled her.

When Cicely's parents got home that evening and Cicely was not there, they assumed that she was with one of their three older children, aged 21, 19 and 16. It was almost 11pm when they realised that she was, in fact, missing; they immediately telephoned the police. On his way home, Straffen bought six pennyworth of chips. He slept soundly and could not understand why the police called at 9am the next morning.

The Batstone case may have been the most straightforward murder investigation there ever was. The bus conductor had recognised not only the girl, whom he'd seen on his route before, but also Straffen. A 20-year-old woman who actually knew Straffen also saw him with the girl. A pair of lovers saw him take the girl into the field, as, more importantly, did the wife of a police officer. It was because she was able to pinpoint the place where she'd last seen them that the body was discovered so quickly, at 8.45 the following morning. "It was obvious to we experienced officers," wrote one of the policemen who found the body, "that something untoward had happened."

When Straffen was taken for interview, he asked, "Is it about the little girl I was at the pictures with last night?" He explained how they'd left one cinema, and gone to the other, and then walked to the field. Straffen added, "She was dead under the hedge when I left her."

That afternoon, he approached a sergeant and said, "Can I speak to you? I want to show you how I did in the first girl." Straffen then described the murder of Brenda Goddard. "She never screamed at me when I squeezed her neck, so I bashed her head against the wall. I didn't feel sorry and forgot about it." Whenever a different officer appeared, it seemed, Straffen would blurt out a fresh confession. "It only took a couple of minutes and she was dead," he told another.

John Straffen was born at Borden, Hampshire, on February 27, 1930. His father was in the armed forces and was posted to India in 1932. The family (John had a brother and a sister) returned to England in 1938, and lived over a cafe in Bath. That same year, Straffen was cautioned by police for stealing a rabbit. In July 1939, he stole a comic from a newsagent's. When he was caught thieving a third time (a purse, on this occasion), he was sent before Bath juvenile court and placed on probation for 12 months. In July 1940, he again appeared before the magistrates for a breach of his probation. In advance of this hearing, he'd been examined by the school medical officer and was certified as a mental defective. He was sent to special schools, institutions for what were then called mentally defective children, in Warwickshire and Worcestershire, and was allowed to leave on March 31, 1946.

He got in trouble with the police three times in 1947. He assaulted a 13-year-old girl in Bathwick, placing his hand over her mouth and saying, "What would you do if I killed you? I have done it before." She escaped, and no action was taken. On September 12, in a fit of rage at a girl he knew, he wrung the necks of five chickens belonging to her father. Prior to this, he had again been arrested on a charge of theft. The stolen property was all recovered for the simple reason that Straffen showed police exactly where he had hidden it. He was briefly remanded to Horfield prison. While there, a fresh medical report was obtained. This reached the same conclusions as the previous report and, on October 10, Straffen was certified as a feeble-minded person and committed to a colony for mental defectives in Almondsbury, north of Bristol. He twice escaped before his eventual release on licence to his parents on April 4, 1951, after which Straffen found work with a market gardener in Bathampton.

On July 10, he was again examined. Electro-encephalograph (EEG) readings taken at a Bristol hospital showed that Straffen had suffered "wide and severe damage to the cerebral cortex, probably from an attack of encephalitis in India before the age of six" (Medical World). The tests were repeated a month later; the results were the same. The doctor who examined him was probably too sympathetic to Straffen's plight, and reached the fatal decision that his licence could continue. Five days later, Straffen murdered the first of his victims.

He was committed for trial for the murders of Brenda Goddard and Cicely Batstone. The case was heard at Taunton on October 17. Dr Parks, the medical officer at Bristol prison, reported: "His [Straffen's] mind is immature and subnormal - he does not realise the seriousness of the situation - he has little realisation of right and wrong and no sense of shame - he is suffering from a disease of the mind, mental defectiveness. In my opinion, he is NOT fit to plead because (a) he does not really appreciate what the word guilty means; and (b) he is not able to instruct counsel, he does not know what they are or what they do." A second doctor supported this diagnosis. A complete record of the court hearing is still available. It runs to just over two pages, and ends as follows:

Mr Justice Oliver: Members of the jury, in this country we do not try people who are insane, so insane as not to understand what is going on; you might as well try a baby in arms. If the man cannot understand through insanity what is going on and is unable to tell his counsel what his defence is, we do not try him. Will you return the verdict that he is insane so as to be unfit to plead, and then I can deal with him?

The clerk: Members of the jury, do you find the prisoner insane so as to be unfit to plead to this indictment?

Foreman of the jury: Yes.

Mr Justice Oliver: He will be detained in safe custody [during] His Majesty's pleasure.

The custody, however, was not that safe. Just six months later, on April 29, 1952, Straffen went over the wall at Broadmoor. It was hardly a dramatic escape; it turned out to have been remarkably easy. While on cleaning duties, Straffen had gone outside on the pretext of shaking a mat. He then used an oil-drum to climb on to the roof of a storage shed, which itself was only 18 inches below the height of the exterior wall. He climbed on to the wall and dropped down the other side on to a convenient fire-hydrant box, and simply walked away. That was at 2.42pm. He walked up the drive of a house belonging to Mrs Doris Spencer, whose husband worked at Broadmoor. He told her that he was a stranger, and she offered him a cup of tea.

"Broadmoor Institution is not very far from here, is it?" remarked Straffen. "About a quarter of a mile," she replied. "Is that all?" said Straffen. "Do the patients escape sometimes?" "Sometimes," she said, becoming uneasy. "Do they ever come down to the village when they escape?" "I think they usually go up the other side of the hill into the pine trees." "Do you think the attendants would look for anyone who escaped?" continued Straffen. "I expect they search everywhere."

Straffen left after about 15 minutes, and walked off in the general direction of Bath. Mrs Spencer naturally telephoned Broadmoor immediately.

Although the press subsequently carried reports that the police and the authorities had instantly thrown a net around the county, the reality appears to have been that Straffen was pursued by two warders on bicycles. He walked for more than seven miles, and passed through the hamlet of Farley Hill. Finally, in Arborfield, he got a lift with Mrs Dorothy Miles. He asked if she could take him into town [Wokingham]; she replied that she'd take him to the bus stop. As she let him out, she noticed the warders not far behind. Straffen said, "Are those police? What are they doing?" He ran off not to the bus stop, but in the direction of the Bramshill Hunt Inn. Mrs Miles showed the warders where he'd gone and, after a brief struggle, Straffen was recaptured in a field behind the pub. It was 6.40pm. He had been free for almost exactly four hours.

At 10.30pm, Alice and Roy Simms, the recently-married mother and stepfather of five-year-old Linda Bowyer, went to the police to report that their daughter had gone out for a ride on her bicycle that afternoon and was now missing.

The escape from Broadmoor was headline news in the next day's papers. Straffen's original court hearing had been a column-filler in many papers; now, his name became infamous. All the papers linked the story of Straffen's escape with the story of the missing child. Her body was found at first light, amid bluebells in a copse behind her home. She, too, had been strangled. The police arrived at Broadmoor at 7.58am, and questioned Straffen at about 8.30am. He said to them, "I know what you policemen are. I know I killed two little children, but I didn't kill the little girl with the bicycle." Berkshire police applied to Pangbourne magistrates for a warrant for his arrest.

On being arrested, Straffen commented, "That is a frame-up, that is." He readily gave his consent for fingernail scrapings to be taken, saying, "They did that to me before. But you will be unlucky this time."

In fact, the police could not take scrapings from the right hand because the nails were bitten right down.

The authorities must have been in a dilemma. The natural course would have been to return Straffen to Broadmoor. Plainly, that could not happen. The communities in the locality were in ferment after Straffen's almost nonchalant escape and his presumed murder of Linda Bowyer. There were questions in parliament.

There was a subtext to those concerns. With the setting-up of the National Health Service in 1948, responsibility for Broadmoor had passed from the Home Office to the Department of Health. Broadmoor Criminal Lunatic Asylum was then renamed Broadmoor Institution. Its superintendent at that time was the reformer Dr Joseph Hopwood, who believed that inmates should be described as patients (the word prisoner was not used), and encouraged links with the local community. Hence, the demands for greater security were now coupled with demands that the tide of reform be turned back and Broadmoor returned to Home Office control. The Prison Officers' Association described the progressive methods as a waste of time and money, and asked for the old, disciplinarian regime to be restored.

So as not to inflame popular feeling still further, Straffen was moved to the hospital wing of Brixton prison, south London. He was brought before Reading magistrates on May 15. The prosecution outlined its case, saying that the murder had been committed between 5.45pm and 6.15pm. Despite a strong challenge by Straffen's lawyer, who argued that it was impossible for him to have committed the crime in that time-frame, the case was sent for trial at Winchester assizes. When the case opened before Mr Justice Cassels on July 21, an inequality in legal firepower was immediately apparent. Straffen's lawyer, Henry Elam, was assiduous and determined, but he was not yet a QC; on the other hand, the Crown's advocate was the solicitor-general, Sir Reginald Manningham-Buller QC, who was known to contemporaries, apparently with good reason, as Bullying-Manner. The Crown's numbers were swelled by the presence in court of the chief constable of Bath, the chief constable of Berkshire and the director of public prosecutions himself. Manningham-Buller first needed to bypass the finding of the previous court and show that Straffen could now stand trial. He called three doctors who, though all agreed that Straffen's mental age was only nine-and-a-half, declared that he was fit to plead, partly on the basis that he knew four of the 10 Commandments.

One of the three, Dr James Murdoch of Wandsworth prison, was questioned by the defence. "In your view," asked Elam, "was he sane?" "Yes." "He is sane although he was in Broadmoor?" "Yes, there are quite a number of sane people in Broadmoor." This remark provoked laughter in the courtroom.

The Broadmoor doctor, Dr Richard Williams, gave evidence for the defence. Asked whether Straffen would know the nature of the act of killing, he responded, "Yes, but not as a normal person." Nevertheless, the judge allowed the case to proceed.

In the event, there were only two strands to the prosecution case, both of which were fiercely contested by the defence. In the first place, the Crown wished to introduce evidence relating to the Goddard and Batstone murders. As is well known, the usual rule is that a defendant's previous convictions should not be disclosed to the jury. In this instance, however, Straffen had not been tried for those offences, so technically they were not convictions. Manningham-Buller asserted that the "striking similarities" in the three murders would help prove the identity of Linda's murderer. The judge acceded to the Crown request.

The other part of the Crown case was that Straffen had pleaded his innocence to police - "I didn't kill the little girl with the bicycle" - before he could have known that a girl on a bicycle had been murdered. The defence protested against the admission of this evidence also, pointing to the national publicity and the likelihood of Straffen either having been told, or having inferred, what had happened.

With these legal arguments having been heard, and the prosecution case underway, the trial was suddenly abandoned. One of the jurors, William Gladwin, was obliged to admit that he had discussed the case in public. The previous evening, he had gone into the Southsea Liberal Club, where his wife was organising a whist drive. He claimed to have been provoked by a guest who, in a most illiberal way, kept asserting that Straffen was guilty as hell and that the trial was a waste of public money. Gladwin said that he "fell into the trap" and retorted that everyone is presumed innocent until found guilty. He added that he had studied the maps of the locality given to the jurors and had concluded that Straffen was not guilty. "In my opinion," he said, "one of the witnesses was responsible for the murder."

All this was reported to the judge, who stopped the trial, empanelled a fresh jury and started all over again. It is difficult to understand why he could not have ordered Gladwin to stand down and continued with 11 jurors. By the time a fresh jury was sworn in, the case had received intense, highly prejudicial publicity. Then, the next day, most newspapers ran on their front pages a photograph of the mothers of the three murdered girls having tea together. This can only have reinforced the public belief that the same man was responsible for all three murders.

Elam, Straffen's lawyer, described the prosecution case as "lamentably thin", but the defence was by now in an impossible position. Straffen, of course, could not enter the witness box in his own defence. "The spectacle of a mental defective giving evidence," explained Elam, "would indeed be a pathetic, tragic sight and one I am not going to ask you [the jury] to see."

The new jury found Straffen guilty and made no recommendation for mercy. He was sentenced to death, and transferred to the condemned cell at Wandsworth prison. Straffen's appeal was turned down and the execution was set for Thursday, September 4.

With time running out, there was widespread concern. How could it be that someone found unfit to plead and sent to Broadmoor could, a mere six months later, suddenly become fit to stand trial? A contributor to the journal Medical World famously argued that it was not the sanity of John Straffen that was in question, but "the sanity of the law". On August 29, Straffen was reprieved by the home secretary, Sir David Maxwell-Fyfe. He was, however, not remanded to Broadmoor but instead sent to prison for life. The Daily Mail considered that "he could be released after 12 or 15 years".

Today, almost 50 years later, John Straffen is still in prison, and still insistent that he did not murder Linda Bowyer. From an observation of his character, one would conclude that Straffen never had the mental capacity to dissemble. An analysis of the evidence in the case, too, suggests his word should be accepted.

The prosecution implicitly accepted part of the defence put forward at the magistrates' court; by the time the case was heard at assizes, it had abandoned its original position and moved the time-frame for the crime to a 5.30-6pm period. This shift, however, only highlighted the weakness of its case.

There is a remarkable passage in the judge's summing-up. Mr Justice Cassels told the jury: "You should bear in mind that when something unusual has occurred and witnesses at a later stage recall it in statements or evidence, they give times which may not always be accurate." Put another way, this was an acknowledgment that the Crown case was predicated on the unreliability of its own witnesses. If they were all giving accurate evidence, then Straffen could not have committed the crime.

In fact, almost all of Straffen's statement was confirmed by independent witnesses. He was seen walking through Farley Hill at 5.35pm by a man drawing his curtains after a children's TV programme had ended. Henrietta Jahn, a childminder at Farley Hill House, saw him between 5.40pm and 5.45pm, and invited him in. When asked if he was out of breath, as if he had been running, she replied, "No." The lady of the house, Mrs Loyalty Kenyon, hospitably brought him tea and biscuits and gave him a newspaper, which he looked through. He left just after 6pm, and was seen by other witnesses before being given a lift by Mrs Miles. Meanwhile, other witnesses saw Linda playing on her bicycle in the road with other children. Annie Saxby said she saw her between 5.30 and 5.50pm. There were other people around at that time, for example a baker completing his afternoon deliveries; none of them saw anything unusual.

As the defence argued throughout, the prosecution case did not fit. Straffen's movements during the vital period were more or less accounted for; there was no opportunity for him to have committed the crime. As would have been expected, there was a complete forensic science examination - nothing was found to link Straffen with the crime, no fingerprints on Linda's bicycle which it seemed likely the murderer had carried.

Straffen had invited discovery both when committing the Goddard and Batstone murders and in his escape from Broadmoor. It was as though, in his childlike way, he wanted to draw attention to what he was doing. It would, therefore, have been remarkable and uncharacteristic for him to have committed a third murder without betraying a trace of his involvement.

The key "similar fact" argument - that there was a common thread between the murders - was advanced in court by the leading pathologist Dr Robert Teare. He had not seen the bodies of the first two victims. All three girls were strangled; none was sexually assaulted in any way; and no attempt was made to conceal the bodies. In other respects, each murder had individual characteristics. The postmortem on Goddard, who was manually strangled from behind, noted five abrasions on the left-hand side of the neck, of which "at least three were crescentic [sic]", and "one crescentic abrasion on the right-hand side". These were fingernail marks. We know that Straffen had no fingernails on his right hand, so that is broadly consistent with the Goddard murder. But not, however, with the Bowyer murder. In his postmortem report, Dr Teare concluded that Bowyer had been manually strangled and observed "a series of four abrasions on the right side of the neck [and] five abrasions on the left side of the neck". In other words, the murder was committed by someone with just about a full set of fingernails. This was evidence that should have ruled out Straffen as a suspect.

At 7pm on the evening of April 29, Alice Tanner, her husband and her brother were at home together in Farley Hill. "We all heard a scream, which I naturally thought were pigs," explained Mrs Tanner. "It seemed to be quite close. It was coming from the copse. My husband and I waited a minute or two and then we decided to walk along the road to see if a child had been knocked down. There was not a soul on the road either way. I could not have helped seeing anyone."

So three independent witnesses all heard what may have been the child's final scream at 7pm - 20 minutes after Straffen had been recaptured.

Arguments about the levels and types of mental incapacity, and at what point criminal responsibility can be established, have continued to this day. In 1981, the attorney-general, Sir Michael Havers, refused to accept a plea of diminished responsibility on behalf of Peter Sutcliffe, the Yorkshire Ripper. Havers insisted on trying him as if he were sane, and thereby created a technical miscarriage of justice. Three years later, Sutcliffe was moved to Broadmoor, where he has been ever since, after the home secretary accepted medical opinions that he was suffering from "a grave form of mental illness". The Sutcliffe trial was, said the Guardian, "a particularly contorted affair, with the prosecution arguing that Sutcliffe was mad, sane and mad (in that order)". A similar description would apply to Straffen's trial at Winchester.

When the Criminal Cases Review Commission (CCRC) was set up, Straffen, who at that stage had no legal advice, submitted his own application. He is now legally represented, and his case is compelling. The issues now before the CCRC include the curious legal process by which he was convicted; the prejudicial publicity at the time of the trial; and whether, even if the judicial process was valid, his conviction for the Bowyer murder is safe. Straffen's case is now being handled by the lawyer Maslen Merchant, who recently visited him in Long Lartin prison. "I asked him how he felt after he killed Brenda and Cicely, and he replied, 'Terrible, it was a terrible thing which I did.' John feels, though, that the man responsible for such terrible acts no longer exists and that after 50 years inside he has paid his debt to society. He says he did not kill Linda, and wants to challenge his conviction. He dearly wishes to be released from prison and to live somewhere quiet. His is an extremely sad case."