How reoffending by child sex abusers has been cut by a quarter

What reduced Jo Clarke to tears was not an encounter with a paedophile or an interview with a victim. It was a police statement in which a little boy described his ordeal at the hands of a sex attacker. "It was the vocabulary, the child using childish words to explain things he didn't understand," Ms Clarke said. "And that just got to me."

What reduced Jo Clarke to tears was not an encounter with a paedophile or an interview with a victim. It was a police statement in which a little boy described his ordeal at the hands of a sex attacker. "It was the vocabulary, the child using childish words to explain things he didn't understand," Ms Clarke said. "And that just got to me."

She is the head of a Prison Service team that recruits psychologists to deal with paedophiles and sex offenders: she has hired 200 in the past 18 months alone. It is a job few would envy - coming into contact with society's pariahs, the men who steal innocence, who abuse children and, in some cases, kill them.

Yesterday, as criticism mounted over the News of the World's decision to continue "naming and shaming" paedophiles, Ms Clarke described the work of dealing with such men and why there is no shortage of professionals willing to try, often at the expense of their own health.

Recalling the day, early in her career, when she read intimate details of the boy's ordeal, she said: "He used children's words to describe what a man had done to him. His mother made the statement. She said she was sitting on the stairs at home with him when he started talking about it.

"It seemed so horrible to have that child's vocabulary sullied by applying it to such a terrible purpose. He was very young. He had never heard the word 'penis'. It just made me want to cry."

Minutes later, however, she had to meet the paedophile.

"It was a shock," she said. "He was an intelligent, articulate man in his forties; he recognised he had caused damage and that he had a problem, but he thought it was not solvable. That was in the early 1990s. We treated him and, to our knowledge, he hasn't reoffended." And that, said Ms Clarke, was why the small army of prison professionals, counsellors and psychologists dedicated themselves to treating sex offenders and paedophiles. Since the introduction of a sex offenders' treatment programme a decade ago, reoffending had been reduced by 25 per cent, leading to 96 per cent of prison service staff who dealt with such men saying it was the most satisfying work they have ever undertaken.

"It means that for every 100 men you treat who would have reoffended, 25 now won't because of the help we have given," Ms Clarke said. "If you assume, conservatively, that paedophiles would have gone on to sexually abuse at least two more children, then that's 50 children you have spared.

"That's why we do this work, for those children."

There are currently about 12,000 men on the Home Office's register of convicted sex offenders. Since 1997, all sex offenders are recorded on the register. They must also lodge their name and address with local police within 14 days of being released from prison.

Each year, between 1,000 and 2,000 offenders voluntarily go on the treatment programme while in prison. Themost serious sex offenders - including child killers - usually end up at Brixton prison in south London, but regimes are in place at 25 other sites, including Albany on the Isle of Wight, Usk in Gwent, Whatton in Nottinghamshire and Channings Wood in Devon.

Offenders are assessed and given counselling by Prison Service staff, usually two or three counsellors dealing with groups of eight offenders. The group sessions typically last about two hours, during which offenders are persuaded to discuss their problems, fantasies and possible causes of their behaviour.

In extreme cases, one-on-one therapy is given and techniques such as aversive conditioning are administered, where an unpleasant experience - such as a noxious smell - is applied to an offender to make a fantasy or inappropriate arousal undesirable.

"We can't 'cure' sex offenders; we don't think in terms of cure, we think in terms of control," Ms Clarke said. "Once you are a drinker or a smoker, you always will be even after you have given up. You have to find ways to keep you off them. That is what we try to achieve."

Yet while the psychological problems of many offenders improves, the stress on the professionals who come into contact with them can take its toll. Problems include alcohol and drug abuse, inability to sleep and sexual problems because of "invasive" images - unwanted memories of things paedophiles have told them.

Penny Buller, spokeswoman on sex offenders for the Association of Chief Probation Officers, said: "You could say it is like feeling contaminated if you have sat for two hours with serious sex offenders, making them state what they have done without minimising it or using glib phrases. You come out feeling sullied, and switching that off is not easy. You have to find ways of making yourself feel whole again afterwards."

For probation workers dealing with paedophiles, the pressure does not cease at the end of the working day. Because sex offenders are released on licence, with strict conditions attached, probation officers have the power to report suspicious or dubious behaviourto the Parole Board, which can have the suspect reimprisoned at any time.

Ms Buller said: "We do have more control than the public might think, but it means that probation officers working with these people can never relax. You can do your work thoroughly, look out for all the signs, but at midnight, when you're in bed, you never know whether they could be reoffending. And if they do, the pressure increases - the officer has terrible feelings of guilt, of feeling responsible. And, of course, there is then an internal inquiry focusing on the offender and the probation officer."

So is there a better way of controlling paedophiles? In the Sixties and Seventies in the former West Germany, hundreds of sex offenders were physically castrated; they had their testicles removed to dampen sexual desire.

But today an offender could be physically castrated one day and buy replacement hormone therapy on the black market the next, according to Dr Russell Reid, a consultant psychiatrist at the Hillingdon Hospital in west London.

Dr Reid is thought to be the only psychiatrist in the UK currently practising chemical castration, the administration of drugs to dampen libido. For the past 10 years, two of his patients have voluntarily accepted monthly injections of goserelin, trade-named Zoladex, an anti-testosterone agent normally used in the treatment of prostate cancer As a side-effect, goserelin also switches off sexual desire. One of his patients had been given the choice of taking the drug or going to prison for an extremely long term after barricading himself into a room and attempting to rape three boys.

"I am not evangelical about the treatment - there are other respected psychiatrists who are opposed to it - but it has worked for these men," he said. "If you consider that sex offenders are mostly compulsorily driven because they are hypersexual - they may think constantly about sex and require some kind of sexual gratification 10 or more times a day - then they are driven by their penis to find a victim.

"This drug switches off that process completely. However, my patients requested it; full informed consent must be given. There must be offenders out there who would be happy to undergo this treatment, in conjunction with counselling, so they can live a normal life. But there will be others who do not want it, and you can't force them.

"Psychotherapy simply doesn't work for paedophiles. They have terrific powers of self justification and denial. They live in some bizarre fantasy world. Most paedophiles I have come across seem to have the same mental age as the children they abuse."

Whichever argument wins the day, public loathing of paedophiles will continue. In February this year, William Malcolm, a notorious child abuser, was shot dead on his doorstep in Manor Park, east London. In the first four months of the inquiry, the police did not receive a single call from any member of the public offering information to help catch the killers, two men seen running away. If anything there were celebrations.

When The Independent told Malcolm's brother, Andy, who lives in France, that no one was co-operating with the police, he was delighted. "The killers are welcome in my house any time," he said.