Ed and I reclined across the taxi's seat as it swerved through a stream of tail- lights, cutting across a bubbling exodus from central London towards HMP Wandsworth. This would be my first visit since 1978, when a friend was incarcerated there. I remember my shock and revulsion upon going to see him, only to be met by a monosyllabic ashen-faced stranger who smelt funny and never smiled. His broken hand, he insisted, was the result of an accidental fall down a flight of stairs. His mother and I were unconvinced.

Arriving with seconds to spare, we were ushered through the metal detector. It went off. 'Anything metal in your pockets?' asked the guard. 'Only this gun,' I replied, removing a bulky handful from my jacket. He froze for a second before emitting a tight, nervous laugh. I put my keys on the table.

Forget about crush bars and velvet-draped foyers. There is nothing like a prison exercise yard to stimulate and sharpen the senses for a night of drama. As a bonus, Wandsworth is the only jail in Britain to retain its gallows. We walked across a bare tarmac space enclosed by a towering mesh fence, its upper edge festooned with ribbons of sparkling razor wire. Every last inch was saturated with yellow floodlighting.

On either side stood grim- faced men in peaked caps with alsatian dogs at heel. You can probably hear the metallic footsteps, the gentle clinking of key-chains, the bolts being drawn into place as they escorted us into the gym. 'Norman Stanley Fletcher,' muttered Ed as the doors slammed behind us, invoking Porridge and Ronnie Barker's great comic convict.

Suddenly we were among the genteel, theatre-going middle classes of south-west London. People from Wimbledon, Sheen and Putney, sweetly-spoken, liberal, professional peop1e, dressed in Liberty shawls, court shoes, pinstripes, tweeds and Barbours. Like us, they had come to see Pimlico Opera's production of Guys and Dolls.

Pimlico's prison record is as long as your arm. It began with Le Nozze di Figaro in the chapel at Wormwood Scrubs in 1990. This was just after the trouble at Strangeways, so when the inmates leapt up to give the show a standing ovation, prison officers thought a riot had broken out. Despite this, Pimlico was allowed back in 1991 to produce Sondheim's Sweeney Todd, with several Scrubs' lifers in supporting roles. Last year saw West Side Story at Wandsworth, the critical and commercial success of which secured their current tenure with Frank Loesser's masterpiece, based on Damon Runyon's peculiar little Broadway fable. No fewer than 12 inmates are among the cast.

The claustrophobic nature of the venue - along with 800 guests of Her Majesty, we were safely banged up for the next couple of hours - only served to heighten the dramatic impact of a tight, fast and hilarious production. 'If I was a salad, I'd be splashing my dressing,' sang Mary King as Adelaide, squeezing the line for every last salacious drop.

Andrew Rivera as Nathan Detroit gave a near-faultless display of comic timing and delivery, and must surely be a household name in the near future. And Will Roache, Wandsworth's No 1 gym orderly, who broke the British 60kg power-lifting record in this very room, was suitably intimidating as Big Jule. The dedication of the inmates in the cast, and the professionals who coached them, ensured that their inexperience was barely discernible.

As we spilled out into the glare of the floodlights, I scanned the audience once more. Were they slumming, enjoying a night out in others' misery, as uncharitable friends had suggested? Some were seasoned theatre-goers, others perhaps were social workers. Most of them, though, were local people having a fun night out.

'Peoples' motives in coming here are not for me to judge,' said Sally Taylor, Pimlico's general manager. 'But I think it's good for people to see the inside of a prison. I'm sure there are a few who come just to look around, but provided the show stands up it doesn't really matter.'

What of Pimlico Opera? 'Of course, our motives are not entirely altruistic - this is an expensive production and we have to put bums on seats - but hopefully we encourage the artistic nature of the inmates who work with us. What it has taught us is that prison should be an opportunity to show people another side of themselves, that crime doesn't have to be a way of life.'

As we stood waiting to be released, Ed struck up conversation with one of the cast. 'I just take them as they come,' he said, referring to the inmates. 'Most of them seem like decent people. Mind you, one bloke turned out to be a rapist.' Had he found it difficult working with, er, someone like that? 'Oh, I never saw him again. I think the other blokes found out and he 'withdrew'.' We emerged into the damp air, and saw the mist-shrouded Citroens and Volvos drift into the soft bosom of suburbia. It was good to be free again.