Grendon Prison in Buckinghamshire is an unlikely bastion of old-world courtesy and so, as we begin a discussion group in G-wing in Britain's only self-avowedly therapeutic jail, I have to be reminded by Roy, one of the inmates, first to go round the room to give everyone the chance to introduce themselves properly. It is a small detail but one that goes to the core of Grendon's community ethos. This begins for new prisoners when they arrive at the main entrance and officers tell them that here – unlike any other penal institution they've ever been in or ever will be in – everyone is on first-name terms.

I offer a short spiel about coming to observe a day in the life of a jail which is nearing its half century but remains an anomaly in Britain's overcrowded prison estate, which is perpetually under threat from Ministry of Justice bureaucrats who favour "Titan" jails run by private contractors where inmates are stacked high and costs kept low, but which has just picked up the annual Longford Prize as, in the judges' words, "a beacon of hope for the Prison Service because of its proven track record in cutting reoffending and promoting the principles of rehabilitation".

The group listens to me in polite silence. Roy is the next up. He has been in Grendon, he announces matter-of-factly, for the past two-and-a-half years of his life sentence for rape and murder. Then comes Andy, who sexually abused and then murdered his wife and daughter. Tony is also a double murderer, but has, he adds, concurrent sentences for downloading pornographic images of children. Neil is a convicted paedophile. And so it goes on. Each man speaks in an almost flat voice, as if acknowledging their crimes are what define them – which, of course, within a prison setting they do. But there is a slight tentativeness in there too, as if they are still learning to say out loud and take responsibility for the details of their offences. Which is where the therapeutic approach of Grendon comes in.

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As the introductions proceed, the pen freezes in my hand. It won't write what I know needs to be recorded in my notebook. It is all that I can do to keep a blank expression on my face as the toll of death and abuse mounts in this small and suddenly claustrophobic room. This is a challenge for anyone who rejects on principle the popular prejudice that sex offenders are monsters who can never truly be rehabilitated and who should therefore be locked up with the key thrown away. As liberal theory comes face-to-face with real offenders and real crimes, a part of me instinctively recoils.

It is a fleeting insight, one of the prison officers remarks to me later when I admit to that moment of horror, into the challenge that daily faces them at Grendon. The officers are a central part of the community, working side-by-side with therapists, psychologists and facilitators. In group sessions, they don't stand at the door, as in other prisons, maintaining order and turning keys, they participate. Security and therapy are both part of the job description as the notice above the front entrance makes plain – "Her Majesty's Prison Grendon – Therapeutic Communities".

Many of the officers have been here most or all of their working life. "Why did you want to be a prison officer?" I ask Pauline, the officer in charge of F-wing, where new arrivals are introduced to Grendon. "I didn't," she replies without a beat. "I wanted to work at Grendon."

ntil the mid-1980s, Grendon didn't have a governor. As a therapeutic community, it was headed by a Medical Superintendent. Then its senior management structure was brought into line with the rest of the prison system by the Thatcher government. Many feared a dilution of the idealism that had inspired Grendon since it opened in 1962 on a gentle hillside of chalky farmland outside Aylesbury. And have gone on fearing for it ever after, particularly since its cost per each one of its current 190 prisoners, at £42,000 a year, is about £7,000 above the national average for other category-B high-security jails in the system. But survive it has, because it works.

Successive academic studies have shown that re-conviction rates among "Grendon graduates" are lower than across the prison population as a whole. When you bear in mind that Grendon is often dealing with some of the most serious and damaged offenders in the system – for whom every other option has failed and who might otherwise have to be sent to secure mental institutions at a cost to the taxpayer of £250,000 per year – its track record is nothing short of remarkable.

"Ever since Grendon opened," acknowledges Peter Bennett, who has been governor since 2002, "there has been a belief within the therapeutic tradition that it is vulnerable and also that it declines year on year. And Grendon does feel itself isolated because it is very different from the mainstream of the prison system. And the system does sometimes have difficulty in understanding what Grendon is about."

Anxious not to fall into the trap of special pleading for Grendon – something that appears only to exasperate other governors and ministers – Bennett describes his role as that of a broker, helping a system that seems ever more interested in retribution and punishment to cope with and support Grendon, but also reminding his own staff that their workplace is still a prison and has to conform to directives from the Ministry of Justice, however much they resent such interference.

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The prison operates as a therapeutic community round the clock. In other jails, there are therapy sessions, even special therapeutic units. Only at Grendon, though, does therapy pervade every minute of every day. Its programme, says the director of therapeutic communities, Michael Brookes, is "soundly grounded in psycho-dynamic theory and practice".

On weekdays, inmates, officers and therapists spend their mornings in groups. Tuesdays to Thursdays, the 40 inmates on each of the five wings split into groups of eight for intensive work, looking at their upbringing and their crimes. These sessions include art therapy and – something unique to Grendon – psycho-drama. On Mondays and Fridays, the whole wing community gathers.

I'm sitting in on the Monday meeting on G-wing. There is no segregation at Grendon, but offenders are grouped according to their crimes so that in therapy groups they will have common ground. G-wing is for sex offenders.

My participation in the meeting – and it is stressed that in a community there can be no observers – has been sanctioned not just by the prison authorities but also by the inmates themselves who, in this democratic forum, chaired by one of their number, vote on all matters. If someone has been acting up on the wing, missing therapy sessions, or failing to pull their weight in the community life, it is raised, discussed and – if necessary – sanctions are decided by a show of hands. If all else fails, the Wing Meeting can vote an inmate out of Grendon. It is all part of the commitment to encouraging prisoners to take responsibility for their lives – past, present and, hopefully, for those among them who will one day be released back into society, future.

The colourful rugs and artwork on the walls of the room where we gather take the edge off the clinical feel of the place, but only just. Mixed in with the prisoners are the four officers on duty on the wing, the full-time psychologist, Geraldine Akerman, one facilitator and one therapist. Proceedings kick off routinely enough with a vote on whether to spend the wing's social fund on getting some Calgon tablets for the washing machines, but the main topic for debate is how more reticent prisoners can be afforded greater opportunities to speak up in small and big groups.

There is, in this forum, no coyness about naming names. It is simultaneously both supportive and exacting – the most exacting meeting I can remember attending. Bruce, a wirey Liverpudlian with a shaved head, wearing a black and orange tracksuit, points out one of the quiet ones, Noel, an older, chubby man in a pale blue T-shirt sitting with his eyes fixed on the floor. "Noel, you're one who doesn't say much at Wing," Bruce begins. Noel's folded arms visibly tighten around his body. "How could we help you to come out of your comfort zone? Because that's what we are here for, isn't it?" Bruce's tone – in contrast to his combative appearance – is concerned.

Noel puts his hands down between his knees and begins to rock as he speaks. "I'm not too bad when I get started," he says, still looking down.

"But what can we do to get you started?" Bruce persists.

"What you're doing now," Noel replies. "Put me on my toes."

Tom, a few seats further along in a blue fleecy top and tracksuit bottoms, joins in. "I used to find it hard to speak too," he offers, "but we've come here to find a voice, haven't we? A lot of us in our lives haven't had a voice. This is our chance."

It crosses my mind that when they committed their crimes they had a voice, but it seems inappropriate to say it aloud. When I mention it to Akerman later, she tells me I should have. "They are used to us challenging them all the time."

The language of the meeting is a curious mixture of politeness and street speak, swear words and therapy talk. Towards the end, Nigel, a slight, carefully groomed man in his twenties, takes the floor. "I'm uncomfortable that we haven't said anything this morning about two of our community who are in trouble."

Everyone else in the room apparently knows who he is referring to. Living at close quarters, five cells to a landing, they would inevitably know each other's business, but here that curiosity is overlaid with compassion. Jeremy, who has self-harmed the previous night, is trying to hide the dressings on his arms. Two places to his right, a faraway look in his eyes, sits Sid, who has just learnt of his son's death. "Do either of you want to share anything?" asks Gary, the chair.

Sid snaps back into the moment. "I just want to say that life goes on. I'm not going down the road to self-pity. But I'd like to thank you all for asking." Jeremy is more agitated: "I don't want to talk about it," he states several times. Voices pipe up, gently urging him to share, but he is adamant, angry even, and no one tries to force him.

After the two-hour meeting, inmates, officers and therapists mingle in the wing corridors to mull over what has happened. On a row of chairs, I spot Sid. Bruce is next to him with his arm round his shoulder.

The staff office door is always open and prisoners walk in and out freely, sharing a joke, or checking on details of what they will be doing that afternoon. All do some form of activity – whether it be in the kitchen, the gym, the medical wing or in education – but again this feature of every prison routine has a distinctly therapeutic twist to it at Grendon.

Instead of being escorted to and from their various workplaces, the prisoners make their own way. It is called "freeflow" and makes absolutely real the commitment to no segregation. Despite the serious crimes of many inmates, levels of bullying, violence, drug use and intimidation are exceptionally low in Grendon.

Each prisoner has to seek and receive approval from the Wing Meeting for whatever form of work or activity they want to undertake. It has to chime with their therapy. "I'm on healthcare," Bruce explains. "A lot of my crimes were linked with drugs and my attitude to women. On healthcare, it's mostly women staff and I have to be trusted with the drugs that are about."

Though I cannot join any of the smaller therapy sessions, a group of 12 G-wing inmates do agree to sit and talk with me. Which is where we start going round the room with each one detailing their crimes. Every inmate at Grendon has to apply to come here. Often there is a waiting list. Some of those who make it to the induction wing are then rejected as unsuitable. So what attracts them?

Jim is a convicted rapist and murderer who has been at Grendon for two years. His puppyish face belies his appalling crimes. "I felt as if I had 10 pieces to a 20 piece jigsaw puzzle," he says, "and I wanted to go back over my life and find the rest. In other prisons Grendon has a reputation for being a cushy option, but there's nothing easy here. The therapy I have been doing here has turned my hair grey."

Like Jim, Stan, a lifer, 20 years into his sentence, does psycho-drama therapy every Tuesday. Here inmates attempt to put themselves in their victims' shoes. "I've been here for three-and-a-half years and was having a really bad patch just before Christmas. I was contemplating taking my own life. Then in psycho-drama, they challenged me to look at my crime. I had to watch myself doing what I did. And I had to act out being my victim. It made me retch into a bucket." There are tears in his deep-set eyes as he speaks.

Not everyone in the group is so positive. Jerry wants a transfer, he says, even though he's only been here 13 weeks. "I just don't have the memories they want you to have," he says. "I've gone as far as I can with this therapy. I'd rather have more time behind the door [locked in his cell] at another prison than all this stress."

The others listen respectfully as he damns the programme into which many of them have invested four or five years of their life. Jim gently suggests that there is a cycle of highs and lows in therapy, and that Jerry may just have reached a particularly challenging place. Jerry, though, is having none of it and walks out to go to lunch.

Nathan, a serial rapist, is up for parole soon. Does he believe what he's discovered about himself in therapy will stop him reoffending? "Grendon will never leave me," he answers emphatically, with the zeal of a recent convert. "Even if I leave tomorrow. It gets so deep inside you. It will have an impact on everything I do from now on."

It may sound like telling me what he thinks I want to hear, or even trying out a good line for his parole board hearing, but at a time when reoffending rates are at record levels, Nathan's hopes for himself are, according to academic studies of Grendon, soundly based in the experience of others who have emerged from this therapeutic community and rebuilt law-abiding lives. The humane and challenging regime at Grendon has clearly given them something and – since it is the rest of society that their crimes impact on – ultimately made our lives safer too.

The names of offenders have been changed