But when Tim Devlin first told me of the case, he insisted there was no murder, merely a tragic accident. The elderly woman could - despite the odds against it - have walked to her death down a lonely country lane and fallen into the river in the dark. The theory seemed implausible. Equally implausible, maintained Tim and his wife Angela, was that Sheila Bowler, pillar of the small Sussex community of Rye for many years, would have committed murder.

He outlined his theory in detail on September 20 1994 in the law pages of The Times. It was explored further that night on television by Channel Four in its Trial and Error programme. It was the first public suggestion that Mrs Bowler, dubbed the Black Widow by the tabloids after her conviction in July 1993, might after all be innocent. Her first appeal in the wake of the article and television programme failed. But Michael Howard, then Home Secretary, referred the case back to the Court of Appeal and in July 1997 the Lord ChiefJustice, Lord Bingham of Cornhill, quashed her conviction and ordered a re-trial. After four and a half years in prison, Mrs Bowler was released on bail.

I became intrigued by the case. The Devlins would not champion such a cause lightly. Their conviction in Mrs Bowler's innocence prompted me to drive to Rye and with them walk along the remote winding lane down to the River Brede where Aunt Flo met her death. Seeing the 500 metres she would have had to wander in the dark from the car (abandoned by Mrs Bowler as she sought help for a flat tyre) I found it impossible to believe that a frail, semi-demented woman of 89 years, said by everyone to be unable to walk more than a few steps unaided, could have managed the journey.

Then there was Mrs Bowler herself. I felt certain that after interviewing her I would have a firm feel as to her innocence or otherwise. At her home she was busy preparing for her forthcoming trial in just a few days' time. She had been back there since being released on bail a few months before. Despite the exceptional circumstances - the mantelpiece bore rows of good luck cards - it was easy to glimpse what her life must have been four years earlier. Tidy, ordered and utterly respectable.

She, too, seemed remarkably composed, despite confessing to nerves about the trial the next week. One could see why the police officers, visiting her after the discovery of Aunt Flo's body, might have been suspicious, thought her behaviour odd for someone whose relative had just disappeared. She had carried on as normal, making biscuits. She was not an easy interviewee, hard to pin down and much of the time preoccupied with hunting for her new kitten in the garden or making telephone calls on the finer details of her re-trial. She talked without anger or bitterness - even with humour of her time in prison. Her emotions were under control. It was by no means impossible to see how the Prosecution, at her first trial, could have depicted her as 'cold, calculating and callous.'

The Devlins readily admitted that Sheila Bowler's own personality had helped bring about her conviction. So why had they embarked on the campaign to free her? They had known Sheila Bowler for some 15 years before the death of Aunt Flo, but more as acquaintances than friends. Their daughters were at the same primary school in Rye. Four factors however convinced them of her innocence: first, she doted on her daughter Jane and was passionate about her musical career. If she had planned to murder Aunt Flo, she would never have done so in the middle - as it then was - of Jane's music degree finals. Second, she was an intelligent woman: the 'murder plot,' if such it was, was doomed to fail because Mrs Bowler herself was aware that everyone believed the old woman incapable of walking any distance. So the finger of suspicion would immediately fall on her. Third, she was a compulsive talker with an abrasive personality, she easily offended people and would have infuriated the police. But fourth, beneath her brusque front, she had a heart of gold - many local people testified to her good deeds. Above all, as the Devlins describe in the book, she had cared lovingly for Flo and her sister Lil for many years and was extremely fond of her.

The first trial had formulated the case in this way: if Mrs Bowler did not kill her aunt, then who did? The idea that there might have been an accident was never postulated. As part of their campaign, the Devlins undertook lengthy research to show Aunt Flo was not the querulous, fragile old lady content to sit in a chair in the old people's home where she lived. She was by nature an explorer, a traveller. She took off in her 60s, flew to New York, hailed a cab and demanded to be taken to the notorious Bowery district to look around. In her 70s she set off for Canada, where she knew no one, and spent two months back-packing across the country, staying in youth hostels and returning as a steerage passenger in a Russian ship. In her 80s she joined the Bowlers on a family holiday in Germany and insisted on taking a trip in a ski-lift.

More significant, perhaps, was the evidence brought forward that elderly people thought unable to walk more than a few steps can, and do, in situations of stress, accomplish the unexpected. Professor Archie Young, a leading geriatrician, argues it is entirely possible that Aunt Flo could have walked to her death. His evidence was not heard at Mrs Bowler's first appeal against conviction but was significant in the second appeal.

By the time of the re-trial, in January this year, the arguments were essentially polarised: Anthony Glass, QC, a formidable advocate, argued convincingly for the Crown that Mrs Jackson could not have walked the distance to the river. Jeremy Roberts, QC, for the Defence, argued equally persuasively that Mrs Bowler - and he called several witnesses as to her character - could not, would not, have committed the murder. It was a stark choice between two improbable scenarios.

The Crown's case, on the face of it, was forceful: Mr Glass cited a list of factors which seemed to suggest guilt. But, as Mr Roberts eloquently put it, they were all circumstantial. Indeed this was a case, he said, of circumstantial evidence par excellence. In other words, there was no 'direct evidence' to link Mrs Bowler with the crime, nothing, he argued, to show she was in any way involved. No one had seen her at the alleged scene of the crime, nor was there any scientific or forensic evidence to connect her with it.

Above all, in his quiet but compelling opening address to the jury, he insisted that it was not necessary to be sure as to what happened that night to Florence Jackson. The circumstances of her death were likely to remain a mystery to which 'none of us,' he said, 'is likely ever to know the answer.'

The court - judge, jury, lawyers, press and supporters - watched as Mrs Bowler entered the witness box to give evidence. Everyone was mindful that, a widow now aged 68 years, she had served four and a half years and was only too well aware of the consequences of failure at this last attempt to assert her innocence. The remainder of the 12-year sentence could yet have to be served - and if she was sent back, she told me, she would undoubtedly die in prison.

This book recounts how a tragic series of misunderstandings, too-ready assumptions and prejudices led to Sheila Bowler's conviction for murder. It describes the harrowing ordeal for the piano teacher of her time in police cells awaiting interrogation, and of her first weeks in Holloway prison. Angela Devlin's own extensive research into prison life gave her an added insight into what Sheila Bowler experienced. Anybody’s Nightmare also draws on Mrs Bowler's personal prison diaries and prison reports to complete the picture of what she endured - not least the moving letter she wrote to her son and daughter, as she waited pessimistically for the result of her first appeal, telling them to get on with their lives.

On a human level, this book is a sad and moving story. But it also prompts questions about the legal system in which Sheila Bowler believed so profoundly. She never worried, over all the months leading up to her first trial, because - as she put it - she had faith in British justice.

When Lord Justice Rose recently quashed the conviction of a man hanged for murder 40 years ago, he reflected on the dangers of the death penalty in a system where human error is bound to be involved. No such system can be infallible. Anybody’s Nightmare is a salutary reminder of the need therefore for vigilance - and the need, above all, to leave room for doubt.

Frances Gibb, April 1998