Margaret O'Halloran has been married for five years but her husband has still not shared their marital home. Instead, he is an hour's journey away, serving his 25th year in prison for a crime he says he did not commit.

Aged 48, he has spent as many years in jail as he has at liberty. Now startling new evidence may mean Martin O'Halloran can finally come home.

"It is many years too late," says Margaret, "but at least justice will finally have been done."

In May 1975, Britain was shocked by the murder in Birmingham of Thomas Walker. A wealthy hairdresser and homosexual, he was kicked to death after leaving a city-centre pub, his body dumped in a ditch.

Exhaustive police inquiries pointed in the direction of Arthur Langford, a local character known as Pegleg, seen drinking with Walker on the night of his death.

Other inquiries, however, also pointed in the direction of O'Halloran, 26, an illiterate Irish immigrant whom Langford had taken under his wing. Witnesses had identified him as also drinking with Langford and Walker. Martin insisted he had been in London with his girlfriend.

Langford, who at first denied being in Birmingham at all, quickly changed his story, saying he and O'Halloran had been in Birmingham together, met Walker and O'Halloran had killed him. Within days of O'Halloran's arrest, his girlfriend also withdrew her statements and denied she had been with him in London on the night of the murder. Things looked bleak.

After eight months on remand, the trial of Langford and O'Halloran opened at Birmingham Crown Court.

Langford changed his story again and said he had been in London with O'Halloran - who, astonishingly, backed this up. The prosecution, able to exploit changing and conflicting statements, portrayed O'Halloran as a man of unpredictable temper who viciously attacked an easy target, then tried to create an alibi.

But the biggest shock was yet to come. O'Halloran's sister Mary Houlihan, then took the stand claiming her brother had turned up at her house on the night of the murder and confessed to the killing. His fate was sealed. After a three-week trial, he was sentenced to life imprisonment. Langford, denoted by the judge as the weaker member of the partnership, was given just four years for manslaughter.

Poorly educated, with no one to speak up for him, O'Halloran may have been forgotten if it were not for a chance encounter which brought Margaret, a psychology teacher, into his life.

Standing in for a friend on a prison visit, she developed a bond with O'Halloran. Within months, they were married.

"Obviously, I had reservations at first as I didn't really know the full details of the case but, as his trust grew, he started to open up to me," recalls Margaret. "He felt very betrayed and for years he had tried to get the authorities to look again at his case but to no avail. He thought he was lost in the system."

O'Halloran showed Margaret a series of letters written by Langford from his prison cell. The first, penned on the day they were sentenced, read: "Well Martin, as you know, you are my best friend and I hope one day you can forgive me. I did this killing on my own." Another read: "Every day I feel a little more sad knowing you are walking around convicted of a crime you did not do."

Langford had also sent a confession statement to O'Halloran's lawyers but it was taken no further because it was yet another version of events.

But even if true, how could all the other evidence be explained? O'Halloran was adamant that he had been in London with his girlfriend on the night of the murder. His wife believed him but what about the evidence of the other witnesses and his sister? Margaret contacted BBC's Rough Justice programme. Its investigators found nearly all the original evidence against O'Halloran did not stand up to their scrutiny. Langford had died three years before but his daughter, Maggie Phillips, was very supportive.

"She told us her father had maintained consistently for 20 years that Martin had nothing to do with the crime and that he would have wanted her to help us," says Rough Justice producer Jane Rogerson.

Further probing led them to two of the pub witnesses, who now said they did not see O'Halloran that night. One recalled that Langford told him O'Halloran was in London but had lied to police to "get them off his back". They also traced the girlfriend, who had given three statements giving O'Halloran an alibi but then retracted them. She now said that while her memory was muddy, she was sure her first statement would have been the truth.

With many of the witnesses now saying that O'Halloran was nowhere near Birmingham, the team was left with the quandary of his sister Mary, who had said he had turned up with bloody clothes and confessed to the murder. Langford's letters revealed they were having an affair and implied she had been motivated by desire to help him, even at the expense of her brother. The bloody clothes she disposed of belonged to Langford who wrote: "She was in love with me. I'm only sorry she hurt you, your sister." Moreover, Mary's story was also contradicted by other independent witnesses.

With such damning new evidence, the original case against O'Halloran was very weak indeed. But why had he thrown his lot in with Langford in court, claiming he was with him?

As Jane Rogerson says: "In another letter, Langford said he manipulated the forgetfulness of his best friend to provide himself with an alibi. It was to be fatal for Martin but it was the price he paid for friendship."

O'Halloran has learnt to cope with confinement. He has taken up painting and has become something of a father figure to other prisoners, helping them to come to terms with life in jail. Meanwhile, with new evidence, his case has been sent to the Criminal Cases Review Commission, which has the power to refer it to the Court of Appeal. This process, however, can take years - for Martin, it may be years too long. Last year, after two strokes, he became partially sighted and partially paralysed.

"He's a very determined man, which is why he has survived as long as he has but he has realistic expectations," says Margaret. "I only hope, whatever happens, some good comes out of this and he has some time in the world. We must be prepared to put things right, even for just one person."

It was along a rural track, in the village of Bentley in the Midlands, where a wealthy homosexual hairdresser, Thomas Walker, was attacked and killed.

He was repeatedly kicked, then dumped in a nearby ditch, to be discovered later.

The man convicted of his murder, Martin O'Halloran, has always claimed he is innocent. Now a BBC programme, Rough Justice, to be broadcast on Wednesday night, says it has new evidence to support him.

The murder inquiry attracted national publicity. The police decided to target the gay scene in Birmingham, because they were sure the murdered man's sexuality was connected.

Roger Clark, a local journalist, followed their investigation. "There were hundreds of police, and half of them were dressed up in floral shirts and tight trousers," he says.

"They set off around all the pubs and clubs in Birmingham which were known to be frequented by homosexuals, and they just rounded them up."

Two suspects were arrested: Martin O'Halloran, an illiterate Irish drifter, who told police he was in London at the time of the killing, and his friend Arthur Langford, who was seen in a Birmingham pub talking to Thomas Walker on the night of the murder.

Langford at first said in his confession that O'Halloran had been the killer.

Although he later retracted this evidence, it was too late to save the 26-year-old man from a life sentence, as the jury heard more damning evidence.

Langford was jailed for four years for manslaughter; but inside, he wrote letters confessing to the crime.

In them, he wrote: "Well Martin, as you know you are my best friend. I did this killing on my own."

The years rolled by. But a chance meeting with a prison visitor changed everything.

Psychology teacher Margaret O'Halloran married the lifer, and discovered the letters. She took them to researchers at the BBC's Rough Justice programme.

"He was telling me that he was innocent, and he was pointing out to me things in his trial that reminded me of other cases I'd heard," she told the programme.

"I began to check those things out, and I began to check other things he told me out, and they all turned out to be true."

One person they found was the daughter of Arthur Langford. He had died three years earlier. Maggie Phillips was sure that Martin O'Halloran was innocent, and that the letters from her father were probably true.

"For 24 years it was the same thing - he was adamant that he'd done it," she told them. "And he wouldn't say that and stick to it all this time if it wasn't true."

Other witnesses who picked out O'Halloran have also changed their stories - one admitting that he lied to get the police off his back.

The young Irishman has spent most of his adult life in prison. He has one supporter in solicitor Nathan Millard, who believes the case should go to the appeal court. But he says time is running out.

"He suffers from a partial paralysis of his facial muscles, and he walks with extreme difficulty," he says. "Clearly, as he is fragile, he may not last that much longer."

Martin O'Halloran today lives in an open prison. After 24 years behind bars, he is daring to hope that the Criminal Cases Review Commission will decide to take an urgent look at his case.