Last week, Barry Thompson, the prison grass, lied - exposing again the danger of convicting one prisoner on the word of another
EVERYONE in the courtroom was told that Barry Thompson was a liar but no one could deny he cut a most convincing figure in the witness box. It was the fifth day of the most sensational criminal trial of the year and the packed public gallery watched in anticipation as the petty villain was cross-examined by one of the country's most successful defence barristers.

William Clegg - the man who had helped clear Colin Stagg of the murder of Rachel Nickell and who was now defending Michael Stone against accusations that he brutally killed Lin Russell and her daughter Megan - was expected to make short work of a man with convictions for dishonesty and intimidating a witness. But Thompson faced Clegg down, confidently correcting the barrister on matters of prison procedure and adroitly deflecting all attempts to undermine his credibility.

Above all, he would not budge from his contention that Stone, a fellow inmate at Elmley prison in Kent, had spoken to him of the Russell killings. Thompson told the court: "He looked menacing with his eyeballs in the back of his head. He said: 'I made a mistake with her, I won't make the same f---ing mistake with you.'" The evidence helped secure the conviction of Stone, who received three life sentences for the murders of Lin and Megan, six, and the attempted murder of elder daughter Josie, nine.

But last Monday - within 48 hours of Stone's conviction - Thompson admitted: "Stone never said the words I attributed to him. I told the jury a pack of lies." He agreed to make a statement to Stone's solicitor.

The retraction - which seems certain to be a major part of Stone's appeal - prompted the Crown Prosecution Service to order an investigation. There is no forensic evidence against Stone and the conviction largely rests on the words of Thompson and two other prisoners who claim to have heard confessions.

THOMPSON's admission of dishonesty has raised the question over whether prosecutors should rely so heavily on so-called "cell confessions" - the same form of evidence that was discredited in the wrongful convictions of the men who were supposed to have admitted the killing of paperboy Carl Bridgewater in the showers at Winson Green prison, Birmingham. Mark Leech, the chairman of Unlock, the national association for ex-offenders, said uncorroborated evidence from prisoners could not be trusted. "This has got to stop. There has been a catalogue of cases where convictions have been obtained using perjured evidence from prisoners who have ulterior motives and obtain benefits from fabricated confessions."

No prisoner has been more successful in extracting cell confessions than Dennis "The Menace" Wilkinson, a vicious kidnapper and sex offender who has served time in jails all over Britain. Wilkinson has appeared as a key prosecution witness in at least eight major criminal trials, where he has alleged that fellow prisoners confided in him about serious crimes. He has been used by police to give evidence against more than 20 men who have been jailed for a total of 90 years.

Wilkinson was first exposed in 1984 after he made his own confession in taped conversations with David Capstick, a private detective and former police officer. Mr Capstick said: "He was nothing less than a professional witness."

Among those Wilkinson helped to jail were Daniel Vaughan and John Haase, who were accused of being part of the notorious "Transit Mob" armed robbery gang. Both men had long been targets for Merseyside police. Despite the fact that Wilkinson was being held in a different part of the jail from the alleged armed robbers, he claimed that Vaughan had admitted the robbery of a post office van "during conversations in the last few weeks". His evidence helped jail the pair for 13 and 14 years respectively, in 1982.

When Wilkinson himself appeared in court later that year accused of kidnapping, torturing and sexually assaulting a man, police attended to praise his evidence in the robbery trial. The judge sentenced Wilkinson to just six years. Wilkinson was later interviewed in Wakefield prison by Paul Baker, the solicitor for Vaughan and Haase.

WILKINSON told Mr Baker that he was bitter at his treatment by the police, who he claimed had promised him a new identity in Canada.

Mr Baker took a statement from him and contacted solicitors from other trials in which he had been involved. None of the convictions was overturned.

Extraordinarily, Wilkinson's work with the police did not stop there. Seven years later, the most notorious gangster in Scotland, Paul Ferris, was arrested and charged with the gangland execution of his bitter rival, Arthur "Fatboy" Thompson. Wilkinson, who had by now been given a new identity of Dennis Woodman but had been jailed again for the attempted abduction of a Scottish farmer, was moved alongside Ferris in the segregation unit of Barlinnie prison in Glasgow.

During what became the longest murder trial in Scottish criminal history, Woodman/ Wilkinson appeared as a key prosecution witness, claiming Ferris had admitted the murder. But his credibility was torn apart by Donald Findlay QC, the leading Scottish criminal advocate who was aware of Woodman's past. In desperation, Woodman swore he was telling the truth "on the ashes" of his dead children. But his wife was summoned to the witness box to say the youngsters were alive, well and waiting in the court foyer.

Ferris was cleared, only to be jailed last July for gun-running. Woodman was moved to the protection unit at Peterhead prison and was released last year, reportedly with a pounds 20,000 price on his head from Britain's underworld.

Woodman is not a one-off case. It is a sad fact that prisoners in all jails habitually make false accusations about fellow inmates in an attempt to ingratiate themselves with the authorities. Tom Robson, of the Prison Officers' Association, said Thompson's retraction had not been unforeseen by staff at Elmley prison. He said: "It's no surprise to anybody that inmates come up with stories and then retract them. It happens all the time."

Dr David Wilson, who served as a governor at five prisons, dealt with tens of thousands of adjudication cases against inmates who were alleged to have broken prison rules. "I knew that I could never rely on the word of a prisoner to convict another prisoner," he said. "They would make allegations to try to gain an 'in' with the prison staff which might lead to them being given a cell change, a better job, parole or a transfer."

Dr Wilson, now a leading criminologist based at the University of Central England, said that because prisoners felt powerless they tended to give the answers they thought the questioner wanted to hear. "They alter their stories according to whether they are talking to police, the prison authorities, other prisoners or their own families."

The idea that the word of prisoners should be accepted as the key evidence in a criminal prosecution like the Russell case was, said Dr Wilson, "extraordinary". He said: "We have got to separate ourselves from the emotional side of this particular case because there are some fundamental principles at stake here."