He testifies as an expert witness - analysing the evidence presented by the prosecution and assessing whether it proves what the prosecutors claim it does.

His expertise has been accepted by the courts and appears to have convinced juries and appeal court judges: in every one of the 20 cases he has been involved in over the past two years, the side on whose behalf he has given evidence - and it is almost invariably the defence - has won.

Two weeks ago, Dr Thompson found that the door to his home had been smashed in, his house searched, and his computer and many of his files seized. The same happened in his office at Reading University.

The raids were organised by Thames Valley Police.

Officers asserted they had information that Dr Thompson was in possession of child pornography and obtained a search warrant from a local magistrate before entering his premises.

Unfortunately, the police did not tell the magistrate that Dr Thompson was an expert witness in a myriad of child sex cases.

Detective Sergeant Kate Ford, of the Marlow Child Protection Group, who supervised the raid, explained that omission to Dr Thompson's solicitor by saying that: "Dr Thompson claims to be an expert witness, but he is not on any expert witness list we checked."

Dr Thompson is baffled by that claim. "I am on the Home Office website as an expert who has been consulted on the sentencing of paedophiles. I am a practising associate of the British Academy of Experts, which is recognised by the courts as an authenticating body.

Experts employed within the criminal justice system and academics conducting research are two of the six categories of people specifically singled out by the Child Protection Act of 1978 (and its amendment of 1988) as having a statutory defence against any criminal charge of possessing child pornography.

Dr Thompson and his lawyers will argue in the High Court next week that it is extremely doubtful that the search warrant should not have been authorised because Dr Thompson is an expert.

"You can't be an expert witness without having to evaluate this sort of material. It is part of my job," Dr Thompson continued.

"I am frequently asked for an opinion of whether pictures constitute child pornography: often the relevant pictures clearly are obscene, and I will advise them that the pictures cannot be defended. Clients often change their pleas as a consequence."

Dr Thompson has given lectures to police officers in which he has explained the anomalous position that he and other experts such as him occupy in relation to the Child Protection Act.

"We have a statutory defence against any prosecution," Dr Thompson explained. "But it does not stop a prosecution from taking place. Under the law as it stands, people like me can be charged and prosecuted, even though no prosecution should succeed.

"Cork University has a multi-million-pound grant from the EU to study child pornography on the internet. That research couldn't happen at a British university because of the anomalous legal position.

"I have pointed out that it would make sense if the police and Home Office put together a list of experts who were allowed to look at this stuff. The experts could be vetted to ensure that they were not secret paedophiles. I would be quite happy for such a procedure to be in place."

T he police say that, in raiding Dr Thompson, they were simply responding to "several" claims that Dr Thompson was downloading "massive amounts" of child pornography from the internet.

"That claim is demonstrably false," insists Dr Thompson. "And its falsity will be demonstrated the moment they go through my computers."

Dr Thompson has no doubt that he can dispose of the allegation that has been made against him. He doubts that he will even be charged. Still, it will not be so easy for him to restore his reputation: mud sticks.

He has been suspended form his university post while the allegations are investigated. And merely being the subject of a police raid has meant that Dr Thompson has been dropped as an expert witness by the lawyers for several defendants on whose behalf he was due to appear.

"You make a lot of enemies when you point out that not everyone accused of sexually assaulting children is guilty," Dr Thompson notes wearily.

"People say you are an apologist for paedophiles, that you are trying to make the world safe for them. Of course it's nonsense. I want people who are guilty of sexual assaults against children put in prison.

"The problem with the way cases are prosecuted at the moment is that it is often not sex offenders who are being sent to prison, it is innocent people."

As an example, Dr Thompson cites the dozens of men who are now serving terms of between 10 and 15 years for sexually assaulting children in care homes.

The assaults are alleged to have taken place between 20 and 30 years ago. There have been financial incentives for making allegations: the Criminal Injuries Compensation Board will pay out up to £32,000 for sexual assault, and more is forthcoming if a suit can be mounted against the care homes' insurers. Sums of as much as £100,000 have been paid to those who claim to have been victims.

The men who made the accusations that they had been abused are nearly all convicted criminals.

"They usually say that, had it not been for the abuse, they would not have drifted into crime," Dr Thompson explains. "That claim can often be shown to be false: in one case, for example, the individual was in the care home because, even before he was in his teens, he had committed an armed robbery."

What has convinced Dr Thompson of the bogus nature of many of the allegations, however, is his analysis of the evidence that was used to convict the men.

"In 1991," he says, "the Law Lords ruled that allegations could be corroborated by volume - that is, the fact that there were several of them could help to show that they were all true. I can show that this principle is utterly mistaken. Volume, in these cases, does not corroborate anything."

The case of Roy Shuttleworth is a typical example, he says. Mr Shuttleworth worked in Greystone Care Home in Cheshire in the 1970s. He was convicted and sentenced to ten years imprisonment in 1996 for sexually assaulting children in his care.

That was despite the fact that one of those who accused him had a conviction for attempting to obtain money from the Criminal Injuries Compensation Board by deception, and that another was exposed as a liar in a subsequent trial.

"When I went through the witness statements, each one contradicted every other one. They all described Roy Shuttleworth as assaulting them - but each one described a different method of assault. This is, in my experience, quite unprecedented for genuine paedophiles.

"A real paedophile almost always has a single modus operandi which he develops and refines over time, and you find that reflected in statements by their victims: they bribe, they buy their victims something. They don't just suddenly assault, which is what Mr Shuttleworth was described as doing.

"That is not the only anomaly. When you draw up plans of Greystone, you find that what Mr Shuttleworth was alleged to have done was logistically impossible: you couldn't get out of the windows which one 'victim' said he had escaped from, and you couldn't walk naked across the school - as another claimed - in broad daylight without being noticed.

Furthermore, when the surviving duty rosters are compared with some of the dates on which Shuttleworth was supposed to have committed assaults, it became clear that he wasn't even in the care home at the time."

Dr Thompson cites Derek Brushett, who was headmaster at Bryn-y-Don Care Home in South Wales and was sentenced to 12 years for sexually assaulting those in his care, as another example of a case where "corroboration by volume is not corroboration.

"It is in fact contradiction. None of the statements support each other. If you look at them closely, it becomes absolutely clear that Mr Brushett should never have been convicted," he says.

A t least 60 men are in prison as a result of the care home investigations. Bill Thompson has no doubt that most of them should never have been sent there.

"The police went trawling for evidence, and an unholy alliance of psychiatrists, social workers, judges and lawyers has allowed evidence which is demonstrably flawed, if not worthless, to be used to lock people up.

"There is no doubt that there has been child abuse in care homes. Sixty years ago it was rife, and those who made accusations were not believed. But now we have gone to the opposite extreme: guilt at past failures has produced an uncritical desire to believe every allegation.

"That has produced a legal disaster. People are being sent down for 10 or 15 years on evidence that wouldn't even be good enough to produce a court hearing for an accusation of any other crime."

It is not only the care home cases where enthusiasm to secure conviction in child abuse cases has produced serious injustice.

"There are dozens of them in `domestic' abuse cases," says Dr Thompson.

"The police and social services techniques for interviewing children are almost universally abysmal. They are as bad as they were in the Cleveland case. Despite all the reports and criticism, nothing has changed since that scandalous episode over a decade ago.

"In practically every case, they break the guidelines. They make suggestions - in fact, they tell the child what to say. It is appalling.

"To my knowledge, only one police force actually uses a safe interview technique: Derbyshire. If all officers followed their example, I would never be able to find anything wrong with interview statements."

Dr Thompson's outspoken opposition to many of the techniques that have become standard in child abuse cases has created a great deal of opposition. If he is right, however, there are scores, perhaps hundreds, of men now in prison for offences against children which they did not commit.

It is a genuinely "appalling vista" - and one which, as yet, neither the judiciary nor the Home Office seems willing to confront.

Meanwhile Dr Thompson, who has been willing to confront that possibility, finds himself facing the allegation that he is a paedophile.