It was supposed to be a fun day out in London for 14-year-old Margaret, up from Wales in her school holidays. She was visiting Madame Tussauds but was feeling annoyed at her older sister for not allowing her to visit the Chamber of Horrors. Perhaps she seemed too nervous and sensitive to enjoy the ghoulish delights of Crippen and Christie with their brown suits and waxen, murderers' hands. Jostled by the noisy throng of tourists, she walked, instead, round the worthy historical tableaux of soldiers and kings until she found a quiet corner where she could study the guidebook in peace.

As she glanced down the list of notorious criminals, one name jumped out at her. With a rising sense of panic and revulsion, she understood at once why she had been forbidden to enter the Chamber of Horrors. In the pantheon of villains, Major Herbert Rowse Armstrong was the poisoner's poisoner, the artist of arsenic. His effigy stood with the infamous. And he was her father.

The young girl who visited Madame Tussauds in 1929 is now 80 years old. She lives in Bath with her second husband, a retired cleric of genial manner. They both help out in their small way at the local church, but few of those who worship there would guess her secret; how she has spent a lifetime in disguise and in fear of being discovered, all because of something her father did when she was too young to remember.

"I always used to say my mother had died of food poisoning, and my father had fallen and broken his neck," Margaret told me. "Having him in the Chamber of Horrors was one of the things that hurt most. The thought that, for 50 years, people were filing past him, and shuddering at someone I'd remembered as a good father, was excruciating."

But something has happened to make Margaret go against everything she ever learned - that to survive, she must conceal herself. Now, she is talking about her life and her father, and wants everyone to know the true story. Recently, she heard a new account of the case which, for the first time, throws doubt on her father's conviction. "Can you imagine what it's like to go for 73 years thinking your father was a murderer and then discover he was innocent?"

The "Armstrong Case" has become a classic murder story, a familiar genre piece in popular crime-writing. Media interest in the Hay-on-Wye solicitor who fed weedkiller to his shrewish wife, and was tried and hanged for her murder, has been relentless. More than 80 books have been written about the case. Hywel Bennet played Armstrong in the Seventies television serial, Malice Aforethought, and in the past two years alone there has been a radio play, Excuse Fingers, and a multi-million pound LWT dramatisation, Dandelion Dead, screened in February 1994 on prime-time Sunday television. Ironically, it was her dismay at this further television scrutiny that led to Margaret's breaking cover.

In the Fifties, Margaret had managed to stop the BBC transmitting a programme about the trial, but times change. When she wrote to the producers of the proposed melodrama, Dandelion Dead (in which she appears as a little girl), "they said they had committed a quarter of a million pounds in securing actors, and, anyway, it was just history". She wrote again to the directors of LWT and was similarly confounded. Resignedly, she waited for the programme to appear and forced herself to watch it. It confirmed all her fears. "In the very last frame, you hear the noise of a trapdoor and then see the bottom of some brown trousers and black shoes swinging gently in the breeze. That, according to LWT, was dealing sensitively and intelligently with the trial. I thought it was unspeakable."

For some years, Margaret had been in contact with Martin Beales, who was working on a book about the trial after Margaret had given him access to certain papers. She had been reluctant to do so because she was publicity- shy, and, although she liked Beales, was "sick and tired of people making money out of the case". After Dandelion Dead, she changed her mind, and decided to give Beales permission to publish. Beales was making a sensational claim: that he had found evidence which suggested her father had been framed and that his trial had been a farce.

Martin Beales is a solicitor, like Margaret's father. More significantly, he lives at Mayfield, her childhood home in Hay-on-Wye, Herefordshire, and, 70 years on, runs the same legal practice as Armstrong, from the same heavy wooden desk. "When I first walked into the office," Beales recalls, "it hadn't changed at all since the Twenties. There was the same lino on the floor, the same desks and wallpaper." He found Armstrong's name plaque in a dusty recess.

Beales had known the "facts" of the case since moving to Hay-on Wye in 1976. A year earlier, the crime writer Robin Odell had published Exhumation of a Murder, a damning exposition of Armstrong's guilt which was considered to be the last word on the subject.

At first, Beales had no personal interest in the case - quite the reverse, he found it "rather distasteful". Yet something about the Major seemed to dog him. Out and about in Hay, he would bump into people who had known Armstrong and who would bend his ear about the Major. After 50 years, it was still a living issue and Beales soon formed the impression that there was a very strong camp in Hay which had always maintained Armstrong's innocence. He felt destiny drawing him in.

The fateful events which he began to investigate happened in 1921 when Margaret was seven years old. The family, including her brother, Pearson, and sister, Eleanor, was living in Mayfield, a spacious house in the hamlet of Cusop. Her father was a respected country solicitor with bright blue eyes and a carefully waxed moustache. Margaret remembers her mother, Katherine, as "a vague, shadowy figure in bed", who appeared to suffer from clinical depression, including delusions that she had neglected her family and husband and was "liable for arrest". Katherine Armstrong became increasingly neurotic about her health, dosing herself on all sorts of homeopathic cures and quack remedies, and at one point ending up in a mental hospital. In February 1921, after a sudden and violent attack of a stomach condition lasting several days, she died. She was 47, and the cause of death was listed as "gastritis". The wreath Armstrong left on her grave read "From Herbert and the Chicks".

Eight months later, the police began inquiries into an allegation made by Oswald Martin, a rival solicitor in Hay, who claimed that Armstrong had tried to poison him on two occasions - once at a tea-party when, with the immortal words "Excuse fingers", Armstrong handed him a buttered scone heavily laced with arsenic, and again when he sent him a box of Fullers chocolates, some of which were doctored with arsenic. According to Beales, neither of these claims was proven, nor were they subjected to serious legal scrutiny in a courtroom.

The police arrested Armstrong in his office on New Year's Eve, on suspicion of attempted murder. When they searched him, they found one dose of arsenic in his pocket and a detailed description of a treatment for syphilis. Armstrong protested that he had the little twist of paper containing three grains of the poison to kill the dandelions on his lawn.

The inhabitants of Hay were agog. There was initial disbelief, for the Major was a popular pillar of the community, but soon everybody was adding to the rumours and accusations. Two people claimed that Armstrong had poisoned their dogs, someone claimed that the solicitor had offered him a poisoned cigarette. Soon after his arrest, Katherine's body was exhumed.

After the autopsy, the body was found to contain at least three grains of arsenic - about a quarter of a teaspoon, and one-and-a-half times the fatal dose - mostly concentrated in the liver. Armstrong was committed to stand trial for the murder of his wife after it was claimed that he had forged his wife's will and had been leading a roistering life within weeks of her death. Again, neither of these charges was ever proven.

The media, such as it was then, descended en masse on the sleepy market town of Hay, offering £40 for photographs of Armstrong. Margaret remembers a beware of the dog sign was put up in the garden to keep reporters at bay. The trial began on 3 April, 1992 in Hereford's Shire Hall, and it was a sensation. No solicitor had been prosecuted for murder before, and reporters thrilled to Armstrong's cool demeanour in court, perhaps mistaking a familiar ease with this particular courtroom for hubris and sociopathic behaviour.

Found guilty, Major Armstrong was hanged on 31 May. He went to his death protesting his innocence, despite an offer from a newspaper: £5,000 for a death-row confession. Soon afterwards, the children were spirited away from the public gaze.

It is only now, for the first time, that the children's story can be told. Margaret described how their name was changed to Pearson, how she was separated from her brother and sister and sent to live with a family near Cardiff. She was made to wear clothes that made her look like a charity child, and was forbidden to keep any photographs or mementoes of her parents. Even the front page of her Bible - a christening present - was torn out, because it contained an inscription from her disgraced father.

Since her father had no relatives, Margaret was constantly subjected to the spiteful talk of her mother's family. "They loathed my father and said he had had syphilis, which was untrue." After one evening of such talk, her guardian took Margaret aside and whispered, "Don't listen to them - I always thought Herbert was rather a good fellow." It was a small kindness she would never forget.

Her childhood was miserable and disjointed; she was shunted from household to household, sometimes with her siblings, sometimes not. She had never been told her parents' story, although she did know that the subject was not to be raised. On one occasion, when she was about ten, she picked up a copy of a magazine belonging to her Uncle Alf to find that it contained a photograph of her father and an article saying that he'd murdered her mother. "I couldn't believe it," she recalls, "I thought I'd sneak another look when I'd finished my chores, but when I got hold of the magazine again, the page had been torn out.

"I often felt rotten about things," she says. "I wasn't popular at school. When you become friends with someone, there always comes a point when you have to start talking about your family. I knew I must be very careful not to let on or I would be shunned." So she threw herself into her schoolwork. "I carried off most of the prizes at school and some of the parents would say I was a lucky girl. But I'd have swapped all the prizes for a home and parents and somebody there at speech day."

At Cambridge, Margaret read mathematics and then social sciences, in which she studied penology and had to sit through a lecture on her father's trial. During the War, she was a torpedo attack teacher in a flight simulator. She married and had two daughters, but the marriage was unhappy and she took the subsequent divorce very hard. "Because I had no template for family life in my upbringing, I was a failure at it," she says. Meanwhile, she got a job in the civil service and brought up her children.

Then, in the Seventies, interest in the Armstrong case was suddenly rekindled by Robin Odell's book Exhumation of a Murder. Although it was unremittingly hostile to her father (constantly referring to him as a "little man" and indulging in a good deal of cod-psychology), Margaret devoured it. "The book was full of details about my family which no one had ever told me," she says. "It was a bit like a stateless person suddenly getting a passport."

She wrote to the publishers of Exhumation of a Murder to ask for a photograph of her father on horseback, which appeared in the book. One of the book's researchers realised who this petitioner was; he had thought all three Armstrong children had gone to Australia (only Eleanor, who died in 1978, had done so).

It was because of Margaret's letter that Martin Beales was able to make contact with her. One of his assistants approached her for permission to consult the defence files in the case, still stored in a solicitor's office in Hereford, which earlier researchers had overlooked. For some reason, Odell had only used the police files for his book - one reason he was so partisan.

Soon after Beales started his investigations, he arranged for Margaret to visit her childhood home. She was nervous and apprehensive and didn't stay for long, but she was impressed that it was a happy family home with two girls and a boy, just like her own family. A few days later, she gave Beales permission to consult her father's defence files, untouched since 1922, and her official sanction also enabled Beales to access the original files, held by the Director of Public Prosecutions. Margaret was interested in Beales's claims, but was unwilling to give him permission to publish his findings. She still dreaded publicity of any kind. Dandelion Dead changed all that.

Martin Beales's book, Dead not Buried, is nothing short of a rehabilitation of one of the most famous criminals in British history. It makes a case for Armstrong's innocence, or, at the very least, makes it clear that he was the victim of a poorly conducted and unfair trial and appeal. "I'm convinced there are many unsafe convictions in the courts, and this is what makes me implacably opposed to capital punishment," he says. In his view, there are only two real facts in the case: that Mrs Armstrong died of arsenic poisoning, and that Oswald Martin produced a box of chocolates which he said had been sent to him, and which were shown to have been tampered with. Everything else is supposition.

Beales considers the role played by Oswald Martin, Armstrong's rival and accuser, to be suspect, as he does that of Martin's father- in-law, the chemist Fred Davies, who had first accused Armstrong of being a poisoner after selling him arsenic in his shop. Did Davies guess that Mrs Armstrong, known to have been suicidal and to have access to arsenic-based medicines, had taken her own life, and did he decide to use this to frame Armstrong, the only obstacle to his son- in-law becoming the premier solicitor in Hay? It was notable that both Martin and Davies, far from being fted as heroes, were shunned in Hay after the trial and left town within a few years. Martin Beales has no doubts that they hatched a plot against Armstrong. "They took him out," he asserts boldly.

Margaret, meanwhile, has found a new lease of life. She cannot stop telling people who she is and what she has been through. Although she does not intend to petition for a posthumous pardon, she is hoping the stigma will finally leave her family. Her grandson, she tells me, "gets a lot of kudos from his peers" for having a great-grandfather who used to be in the Chamber of Horrors.

Margaret Armstrong will appear with Martin Beales at the Hay Festival on 28 May

'Dead not Buried' by Martin Beales is published by Robert Hale on 30 June, price £15.99