Convicted IRA prisoners are Category A, and each of them has a Category A book. This logs every move the prisoner makes from the day he first enters prison on remand until the day he is released. In the language of prison, he is "on the book".

These books contain a photograph of the prisoner, all his personal details, lists of approved visitors with photographs, lists of approved telephone numbers, and a daily log of the prisoner's movements in the prison.

When a Category A prisoner moves from one jail to another, he take the clothes on his back and the book. The rest of his possessions should follow later.

The Category A books of the six Whitemoor escapees are not available to defence lawyers. They, too, have gone missing

The disappearance of the books was revealed at the second trial of the six men accused of breaking out of Whitemoor on 9 September 1994, which began at Woolwich Crown Court on 13 February, and ended nine days later after the London Evening Standard published pictures identifying three of them as convicted terrorists.

Andrew Russell, the only one not convicted of a terrorist crime, chose to conduct his own defence. Cross-examining Whitemoor's governor, Brodie Clark, Russell asked:

"Is it not true that the reason the Cat A books have been destroyed is because they would have logged the fact that I was in the showers in the Special Secure Unit at exactly the time that the security fence was allegedly being cut?

Governor Clark: "I am not in a position to answer that."

The question goes to the heart of the matter because the men's defence is that, whoever cut the wire fence inside the prison, it was not them. And they are able to make the claim because four minutes of video tape from one of the security cameras is among the missing pieces of evidence.

But the drama unfolding went entirely unnoticed by all but the few spectators in the Woolwich court. By day six it was clear that the court was witnessing an extraordinary spectacle. Senior Prison Officer Comer, who was in charge of the Emergency Control Room, testified that the alarms for the fence could not be switched off in the control room. But a second Prison Officer, Mr Pearson, stated on oath that this was not so. There was a switch in the control room.

The court heard from Mr Comer that the video cameras, which are automatically turned to the spot where the fence alarm is activated, somehow failed to record the vital minutes between 8.09 pm and 8.13pm on the night of 9 September.

The court then heard from Mr Pearson that he and another officer by the name of Morgan were stationed at the video cameras, and had recorded the four minutes. If they did so, the tape has gone missing, and one person whose evidence might have ended the confusion, Prison Officer Marcia Whitehurst, died in a car crash as she was driving to Woolwich to testify.

The jury also heard Governor Clark testify that he had misled the Government inquiry into the escape over the x-ray and metal detection equipment at the jail. The Prison Service had maintained that inadequate equipment allowed visitors to smuggle in weapons and the tools used to cut the fence - including a 2ft long solid steel bolt-cutter.

Once more, prison officers in the witness-box contradicted this story, testifying that metal detection and x-ray facilities at Whitemoor were adequate and that all visitors were put through the procedures. In further evidence, it became clear that the only people who were not searched on entering or leaving the prison were the prison officers.

One prison officer, now retired, admitted that he mended tennis rackets as a sideline and that he regularly took covered rackets in and out of the prison without going through the detection equipment.

The mystery of the missing evidence and the identity of the men who cut the fence may never be solved. Just as the truth was coming out, the prejudicial press report found its way into the Evening Standard, and Mr Justice Kay was forced to stop the case.

This raises two more unanswered questions: in whose real interest was the escape? The securities services, perhaps. And in whose interest is the suppression of the evidence? The Prison Service, perhaps.

Lin Solomon is director of a Channel 4 'Trial and Error' documentary on the case of Danny McNamee.