What do murderers, robbers and drug dealers have to teach children? Quite a lot, it transpires in the course of a day at HMP Coldingley, outside Woking in Surrey. A presentation at the prison - consisting of drama, songs and prisoners’ personal testimonies - aims to be “the best crime diversion scheme in the country”. More than 1,600 teenagers have already attended the Coldingley crime diversion scheme, initiated by lifer Mick Hart, and the monthly “event days” are fully booked until the end of this year.

At 10.30 on a May morning, around 50 young people cluster in the sunshine outside the locked gates of Coldingley. The planted tubs ranged along the entrance and avenue of pink-flowering horse chestnut trees leading up to the prison give a benign impression - which the 18 inmates waiting on the other side of the walls are keen to dispel.

The young people, some of whom have already been on the wrong side of the law, have been brought here by youth offending teams, teachers and youth workers. Mainly boys, some already have convict-style haircuts and a well-developed distrust of authority. At least one has a grey plastic tag around his ankle.

Tony Moth, of Luton youth offending team, has brought six young people he describes as “on the edges of getting into serious trouble”. Fourteen-year-old Jason says: “Some people say prison’s crap, some say it’s all right. I want to see for myself.” Inside the grounds, scrappy wallflowers bloom under the coiled razor wire on top of high fences. Matthew and Lee, both 16, both on probation, look around with interest. “It’s an experience, innit,” says Matthew. “We’re millimetres away from being sent down. It’s a scary thought, not being allowed out or to see your parents.” Lee’s older brother is already in prison, on a drugs conviction.

In a prison dining room, the young people meet Bernard, one of the inmates. With dreadlocks piled into a white net cap, he uses a wallchart to inform them about the types of prison. Category A is for serial killers and terrorists, he tells them, Category B for paedophiles and violent offenders and Category C - like Coldingley - mainly for drug dealers and burglars. The young people sit around the fixed tables under the high windows, eating crisps and drinking Dr Pepper. Bernard reaches the bottom of the chart - young offender institutions. “This is where you’re going to start. A lot of fighting goes on, a lot of stabbings. A lot of bullying. It’s a place you don’t want to be.” In another life, Bernard could have been a teacher.

The main part of the presentation takes place in the prison chapel. Eighteen inmates, all in blue and white striped shirts, introduce themselves by giving their names and the lengths of their sentences. Several are doing life. The master of ceremonies is 24-year-old Avy, who tells the audience: “Today we hope to show you what prison’s really like - prison in the raw. It ain’t all it’s cracked up to be, believe me.”

The play that follows features the story of Joe and Mark, who begin by stealing from a corner shop while bunking off school. The drama follows their developing careers. Joe and “Knock Knock” - the fence - constantly pressure Mark to go for the apparently easy money offered by drugs and crime, but he resists. While Joe drifts into petty theft and drug-dealing, Mark chooses Mandy and a job in a garage. By the end, Mark has a dark jacket and his own car showroom while Joe, just out of prison, has nothing and is begging his old friend for a job.

The storyline has sufficient credibility to hold the young people’s interest. They also have to participate. Between each scene, inmates huddle with the teenagers, urging them to suggest ways that Joe might have been able to resist the pressures that led him into crime. The prisoners and their young visitors then take the floor together to improvise scenes. Prisoner Scott - who obviously spends his spare time working out - is persuasive in his efforts to get Michael to “keep dog” while he steals food for them both. “We can’t go back to your Nan’s for something to eat. She’s 94. You keep him talking, I’ll do the nicking.” But teenager Michael resists his blandishments, and to applause from the floor they go back to his house for pizza instead.

The drama is interspersed by inmates standing up and talking about how they got into crime. One, a serious-faced man in his thirties, says he was eight when he “got into thieving”. For the next seven years he was stealing cars and motorbikes, and at 15 started using violence in his crimes. “One evening, I went out with some other guys and someone ended up being stabbed to death,” he says. He’s 33, and has been inside 18 years, longer than any of them have been alive. “This is where I’ve ended up. Where are you going to end up?” The drama gets progressively heavier. In a chilling scene, newly admitted Joe is put into a cell with two huge men who don’t want him there. The way they intimidate and humiliate him - refusing even to let him sit down - has a horrifying ring of truth. Later, Joe considers suicide in the isolation block he is put into when he’s too afraid to enter his cell.

What is most striking about the presentation is the passion the men put into trying to get the youngsters to learn from their mistakes. Tony, 33, who plays the part of Joe, explains that the event days give him “a sense of well-being”. He says: “Someone could have got through to me when I was their age, with a different approach. All we had in south London was the local bobby.” Another, 29-year-old Matt, has finished a drug rehabilitation programme at the prison and is training to be a counsellor. “If I can help one person, it’s worthwhile,” he says. “I was very selfish before. I’ve had to work a lot on myself but now I’m free inside.”

Another inmate, gold tooth glinting, whispers urgently to a small group of boys. “I’m going to tell you something, and don’t ever forget it. No one loves you more than yourself. Look after yourselves.”

Bob Taylor runs Inside Job Theatre Company, which creates theatre projects with inmates. He helped Mick Hart develop the play. “It came very much from his drive and inspiration,” he says. “It was his means of reparation.” Mick Hart has since been transferred to another prison, but with pound;15,000 a year funding from the J Paul Getty Junior Charitable Trust, the project has gone from strength to strength.

Although the scheme started with the idea of steering teenagers away from crime, the inmates get just as much out of it, says Bob Taylor. “The late realisation was that the inmates were the hidden beneficiaries. They’re unpacking what crime is, and in doing it for the young people they’re doing it for themselves. They’ve probably never reflected and analysed before.”

Forty men are on the waiting list to take part in the presentation and in a pilot scheme. And eight of those currently involved are going for an accreditation in basic youth work skills. “It’s one area where a criminal record needn’t disadvantage them,” says Bob Taylor. “They have a lot of clout as offenders themselves, and understanding of young people.”

Whether the tragedy of these men’s lives can really be understood by the young people is a moot point. The kids tap their feet as inmate Errol - formerly a professional musician - sings: “By now, you should have realised, that prison’s not the place for you,” to the tune of Oasis’s “Wonderwall”. Another song is in memory of Leah Betts, the teenager who died after taking ecstasy, “for the dancer she once was, and the mum she’ll never be”. In this context, the men seem more like missionaries than cons - Errol’s big, melancholy voice fills the chapel and floats out of the barred windows.

After the play, the young people have a closed session with the prisoners. Aware that some young people - aided and abetted by television - glamorise prison life, the men have an urgent desire to warn the children. They tell them that, despite the hard image, every prisoner cries “behind that door, when you’re banged up at night”. And: “It’s fucking shit in here man and YOIs are worse. Every one of us wishes we were your age again. We’re not here to fucking patronise any of you. All we’re saying to you is that you’ve got choices.”

The effectiveness of the presentation in altering young people’s behaviour is difficult to measure, although anecdotal evidence is encouraging. Grant Jack, senior social worker with Hillingdon youth offending team, has brought eight young people here over the past two years - so far only two have ended up in custody. Bob Taylor believes the prisoners are sometimes the only authority figures that some of the teenagers who come here can bear to listen to. “There’s nothing new about the message, but it’s where it’s coming from that’s new. That’s what gets under the young people’s radar.”

The presentation ends with a look at the cells. Errol has volunteered his for inspection. It’s small, with a dismal view out of the window and pornographic posters on the wall. His beautifully executed watercolours rest on the easel. You can’t imagine how Errol’s voice can live in this tiny space. “Doesn’t that do something to their human rights, having to crap in that?” says Jason, looking at a plastic bowl in the “strip cell” when the tour reaches the segregation wing. “Nope,” says the officer, flatly.

The kids say warm goodbyes to the inmates and filter out of the prison back to their minibuses and life outside. Matthew’s verdict? “I enjoyed it. We’ve heard it from the people that know.”

All children’s names have been changed.Coldingley prison crime diversion scheme can be contacted co the governor, Coldingley prison, Bisley, Surrey GU24 9EX. Tel: 01483 476721