Getting into Broadmoor is almost as difficult as getting out. It's not just the mobile, tape recorder and camera that have to go. Watches, phone cards, sticky tape and Tic Tacs are off limits too. On the second of two full-body searches, a security guard discovers a tiny plastic sachet containing a spare button still attached to the inside of my new shirt. "No plastic bags," she barks, confiscating it.

Passing out of the gauntlet of security and into the hospital courtyard, there is an eerie silence, disturbed only by quiet footfalls on gravel and the vague clink of security gates. The towering, wire-topped fences that loom in every direction around the leafy grounds belie the hospital's apparent pleasantness. The Moors Murderer, Ian Brady, the Yorkshire Ripper, Peter Sutcliffe, and the East End gangster Reggie Kray have all been guests inside these walls.

Beyond the courtyard is the redbrick arch of the entrance to the old lunatic asylum. Broadmoor opened in 1863, in the Berkshire village of Crowthorne, as the country's first purpose-made home for the criminally insane. Its hanging clock must once have had a foreboding aspect for hapless "lunatics" offloaded underneath it from horse-drawn ambulances, unlikely to ever again see the outside world. Today, it looks toy-like, almost comical, dwarfed by the colossal, snaking walls that shield it, along with a jumble of newer buildings, from the outside world.

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Newest of all is the Paddock Centre, its smooth, clean curves out of place against the crumbling charm of the Victorian buildings. The £36m centre was made to accommodate some of the most difficult patients a psychiatrist is likely to encounter: sex offenders with Dangerous and Severe Personality Disorder (DSPD). These individuals number among Britain's most dangerous criminals.

Until recently, many psychiatrists had washed their hands of these offenders. Patients with DSPD are not mentally ill, and so technically they cannot be "cured". Their disorder is considered to be permanent: it is, quite simply, a severely anti-social personality. Some are still serving sentences but others served their time long ago. They are held here indefinitely, not because they need treatment for a mental illness, but because they are simply too dangerous for release into the community.

The DSPD programme aims to do what some psychiatrists believe to be impossible: to teach these individuals to control the destructive impulses which make them a danger to the public, and to eventually release them. The project is being piloted at four secure locations nationwide and will cost the taxpayer about £126m over three years.

We're here at the invitation of Tony Maden, a professor of forensic psychiatry at Imperial College, who is the head clinician at the Paddock Unit. He is keen to explain the work undertaken here and to destigmatise its therapeutic techniques. "It's true that you can't 'cure' DSPD," he says. "But whether you can help these people is rather more complicated. Can you help people with terminal cancer? You certainly can't cure them. But you can give them things to help them to live a more normal life."

Professor Maden is a slight man, weighed down at the waist by a pendulous jangle of keys and a personal attack alarm. As we make our way through the Paddock unit's long, primrose-coloured corridors, our dialogue is punctuated by an unrelenting rhythm of slams and bolts. We pass through wave after wave of heavy, reinforced doors which must be unlocked and then immediately locked again after we pass through. From the safety of the nursing station, which sits like a goldfish ball between the two DSPD wards, there is an opportunity for a guilty sideways glance at the patients themselves. Professor Maden gestures towards a group of tubby, unkempt men slumped listlessly on a sofa in front of Escape to the Country.

"Obesity is a bit of a problem here," he admits. Many of the patients receive benefits, because they are no longer serving time, but have nothing to spend them on except ready meals from the Broadmoor convenience store. Some of them, he says, eat two of these a day on top of their three free canteen meals.

Arguably, the extra pounds are the least of their worries. Each one of these men is a serious, violent offender. The nature of their disorder means these patients are typically bullying, manipulative, pathologically deceptive, and highly impulsive. They act recklessly, with appalling consequences, and often fail to feel remorse. Most are paedophiles, rapists or murderers.

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"You never become hardened to the terrible things these patients have done," says Professor Maden. "One patient I treated would assault boys of the age of about six. He'd just jump on them in a toilet or something. Very frightening for the child, of course. He could not stop doing it. He would be in prison for five years – and it's no fun being in prison as a sex offender – and then three months after being released he'd do it again. And if you let him out now, give him three months and he'd be jumping on a six-year-old boy again."

Excited by the appearance of Professor Maden in the nursing station, a patient approaches the glass window, and starts shouting through a small hatch between patients and staff. "Tony? Tony?" He shouts. "I want an interview with you. I want the request form."

Professor Maden smiles apologetically. "All right, All right," he replies without looking up. Bored, the patient walks away, leaving a faint circle of condensation on the glass.

While held in the unit, the patients must complete a range of courses, which aim to teach them to manage their anger and sexual impulses.

"It's not about them lying on a couch and talking freely," says Professor Maden. "It is very much like adult education. It is very structured and directed. They have to complete specific courses and meet certain goals. And if they're not toeing the line, they're out."

But treating these individuals is a laborious process. It can take months – and sometimes a lie detector test – before a patient will start to tell doctors the truth.

"They start off by saying, 'I didn't do it'." Professor Maden explains. "Self deception is practically universal in sex offenders. Then they will say, 'oh, it just happened', or 'I was provoked into it', or 'I was led on', or 'I was drunk'."

Patients are then offered group therapies, which they often attempt to subvert. "That's a daily occurrence", says Professor Maden. "They go off on a tangent, verbally attack other people, deny that they've ever had any problems."

Finally, they are taught how to reduce their risk of re-offending in the community. "In the same way that you tell alcoholics not to structure their social lives around pubs, you would say to a paedophile: look, you can't work as a babysitter."

So what motivates them to complete the treatment?

"Self interest," says Professor Maden. "The key to treating people with psychopathy is getting them to understand that it is in their best interests to do it because otherwise they will be locked up for the rest of their lives. We are dealing with people who are very, very selfish."

Inevitably, the idea of spending large amounts of public money on such individuals has been controversial. The service has attracted criticism for its alleged "cushy" treatment of offenders. In the Paddock unit, the computer lab, football table and indoor cinema-style projector give the impression of a youth club, even if the windowless isolation cells, where patients are locked up when they are violent, tell a different story.

Even more controversial is the prospect of releasing individuals which society has marked as dangerous. When the rapist Lee Porritt was released from the Paddock unit last June, he boasted to The Sun that he had blagged his way through the therapies, convincing medics he was cured, when really he still had fantasies of raping, burning and biting girls aged 11 or 12.

The result was a PR disaster for the DSPD programme. The Sun ran a front-page story with the headline "I'm a psycho rapist ... why did Broadmoor let me out?" and a leader which accused the hospital of "criminal irresponsibility" in releasing Porritt. Patients on the DSPD unit now like to wind staff up by threatening to "go to The Sun" if they are released.

Professor Maden will not be drawn on Porritt's case, although he points out that doctors at Broadmoor do not have the final say on a patient's release. And he says The Sun's criticism of the service was "ridiculous", insisting that it is in the public interest to treat people with DSPD.

"The worst thing for the victim of a sexual offence is knowing their attacker had done it before, and no one had done anything to prevent it happening again," says Professor Maden. "I am interested in victims and I see the programme that we run as part of doing something to reduce the chances of re-offending. Some psychiatrists don't see that as a proper job for doctors. I do." Professor Maden concedes that some of his patients, particularly those who are psychopathic paedophiles, are unlikely to ever be suitable for release into the community.

"If you have these sexual impulses, combined with psychopathy, which is essentially an inability to feel empathy with your victims, that is a very toxic combination," he says. "With some patients, you are unlikely to make any process without chemical castration."

But he insists that, even in these extreme cases, the treatments offered at Broadmoor are a justifiable use of resources.

"The question for me is: do we as a society want to label any specific group as a subhuman species beyond all help? I just think socially that's wrong. Just as it is no longer acceptable to say that someone who has had a brain injury is incurable, with all that that implies, to me it should no longer be socially acceptable to say: you're a psychopath, so nothing can be done for you."

Dangerous and Severe Personality Disorder

DSPD is not a clinical diagnosis. It refers to a group of patients who have a severe personality disorder, who pose considerable risk of harm to others, and who meet the criteria for detention under the Mental Health Act. The term was not introduced by psychiatrists, but by a 1999 government consultation paper.

Within psychiatry, opinion is divided on whether DSPD is a useful term, and on whether therapies for this group are effective. Some medics argue there is no proof that the treatments actually work.

The efficacy of new medical treatments is usually tested in randomised control trials, which compare the progress of patients receiving treatment with a "control group" of patients who are not.

However, because patients with DSPD are so dangerous, some medics claim it is inappropriate to have a control group who are not receiving care.

A recent report claimed that there was still "an absence of evidence from any source for most of the interventions currently used in the programme," due to the lack of effective randomised control trials.

Psychiatrist Professor Peter Tyrer, who has conducted leading research on DSPD, says he found little evidence that the patients were responding.

"Every treatment known to man has been suggested for this group and the costs are absolutely enormous," he says. "And we've got no idea what's working and what isn't working because nobody has really run a control group."

However, if treatments were not available to this group, it would be unlawful to detain them under the Mental Health Act.