They rehearse in the prison chapel. They don't have a decent bass guitar. And it's going to take years before they can get out on tour. 

Mark, the powerfully built, tattooed lead guitarist, is lost in concentration as he chugs out a relentless rhythm splashed with Van Halen-esque squeals and curlicues. Marco on rhythm guitar and Darren on keyboards fill out all remaining middle frequencies, while Jim underpins the whole thing with a steady throbbing bass. They don't have a drummer at the moment and so they're rehearsing with a drum machine. Nelson (the bassist from New Model Army) and I look on approvingly. 

We could be watching any rock band rehearsing on a Saturday afternoon. Except this one writes slightly better tunes. Oh yes, and we happen to be in HMP Blantyre House, Kent. We're here to help the prison rock band hone its musical and songwriting skills, having been asked in as part of Kent County Council's Millennial arts initiative, Marking Time. 

As it says in the prospectus – erm, prospectus may not be exactly the right word – "HMP Blantyre House is unique within the prison system". All of the inmates have been in other prisons beforehand. They've had to earn their place at Blantyre with at least a year of immaculate behaviour in their last institution. In one prisoner's words: "No drink, no dope, and no physical or verbal violence." All inmates speak highly of Blantyre House. 

Blantyre House is the last stop prior to men becoming law-abiding members of the community. Inmates spend from two to four years here and, after a period of risk assessment, are allowed to work in the outside world and spend weekends at home with their families. The system is based on trust and has a re-offending rate which is lower than other institutions 

The band members, all long-termers, are in their late twenties and early thirties. I get the impression that it's not good etiquette to ask people what they've done, but am told quietly that Blantyre's inmates are "mostly big-time drug-smugglers, a handful of murderers – about 20 lifers." When I tell Jim the bassist that I have a friend currently serving time in Highpoint, because of a moment of craziness, he gazes at me steadily and replies: 

"That's all it takes, Martin." 

When Nelson and I arrived for the first of two sessions, Blantyre looked a pretty grim prospect. Its huge wire gates loomed up on the narrow lane and a bitterly cold wind scythed at us as we shivered and rang the reception bell. We reported to the office, where a cheerful woman told us to surrender mobile phones, tobacco or alcohol. It took some time for her to establish who we were and what our business was. 

"Is it about drugs?" 

"I'm sorry?" 

"Have you come to do the lecture about the drugs?" 

"No, we've come to help the prisoners with recording and songwriting skills." 

There's a pause for re-appraisal, followed by: "Oh." 

Though recognisably institutional on first viewing, HMP Blantyre House is actually more like an old barracks than a prison. Fences, rather than walls, surround the place, and in every direction there are views of the Kentish hills and farmland. When we first met Mark the guitarist, he took us up to his cell and made us builder's-strength tea in paper cups. He explained that the lads had been a bit dispirited of late and hadn't been playing as much. He wasn't sure if they'd all turn up. To add to this, we wouldn't be able to use the prison chapel for practice today. We'd have to use an old caravan by the prison's greenhouses. I diagnosed an initial lack of confidence 

Over the course of the morning the band drifted in as Nelson and I broke the ice by jamming with Mark. Mark, it transpired, was the only member who could play prior to being imprisoned. Brought up in a bass-playing family, Mark had had a fair musical grounding and could play a number of instruments. He had taught all the other members of the band to play, he said, because he needed someone to play with. Darren, the keyboard player had received his entire musical education from Mark. And here was the evidence: the names of every note written on the keys of his instrument. 

"He drilled us like bleedin' sergeant-major at the beginning," Darren confirmed. Mark would yell the notes, Darren would read the notes written on the keys and play them, and music of a sort would ensue. Marco, the other guitarist, joined us; last of all came Jim, who, at about six-foot five inches tall and with long blond hair, actually looked like a rock star. Jim, however, insisted all afternoon that he couldn't really play and steadfastly refused to pick up the bass. Finally, towards the end of the session, he relented. He could play. We had a band. 

One of our biggest surprises came when Nelson and I discovered that the prison had enough musical equipment to start a small rehearsal studio. Two pianos, several electronic keyboards, numerous guitars and amps, a reasonable drum-kit. Even a small multi-tracker. Some of the gear had been donated by various prison trusts, the rest was bought from money earned by the prisoners. But they didn't have a decent bass. I asked them why they didn't sell or part-exchange the spare keyboard for a bass: "It's buggered." "Haven't you got an electronics whiz in here who could bodge it?" I asked. A short pause. "The Bomber. He could do it. Yeah, he's brilliant." I asked them who The Bomber was. They told me he was inside for blowing things up. Mainly telephone boxes. Silly question really. 

Now on our follow-up visit, Nelson and I discover the band have undergone a quantum leap in both ability and confidence. The vague homework I'd set them ("Er... try and polish up those lyrics") has been partially done. I still wish to instil in them further the need to write good lyrics. This is where I'm about to come unstuck. 

I put a cassette of Ray Davies' "Waterloo Sunset" in the tape machine and begin to talk about the perfection of its lyrics. I am interrupted by Jim, who informs me that the song is rubbish. I have never heard this opinion expressed before and with my mizzen mast unexpectedly shot away, I ask him: "Then what d'you reckon is a good lyric?" "Dylan's 'Subterranean Homesick Blues'... lots of stuff by the Stones."Marco tells me later that Jim is a bit of pop mastermind and spends all his time reading. 

It being Sunday, we can have the chapel. Nelson has set up a rudimentary recording system in there, and band and drum machine are firing on all cylinders. Song after song emerges, which Mark sings in a convincingly throaty bark. Jim has got his new bass. Darren isn't looking at the notes so often. Nearly all of the songs are melodic with strong ideas. They want to go into a proper studio as soon as possible. There's not a lot else Nelson and I can do. Our work is done here and the session ends. 

The boys from Essex shake hands with the boys from Kent. One or two are due for parole in a few months. We wish them luck. "Try and read some poetry every once in a while," I plead vainly. Would I dare to say this to Lemmy or Metallica? 

We drive through the big wire gates of HMP Blantyre, down the lane and out into the winter dusk. Nelson is due to go on tour for two months. I have a lot of work to do. We feel lucky. For the first half hour of our journey back to Essex we don't say much.