Whenever the camera stops rolling, dozens of hired hands suddenly swarm on to the set like mechanics in the pit lane at Silverstone, the most welcome of them being the man who wheels in and cranks up the instant heating system - space age gas-burners that roar like two jet-propelled hair-driers. The bell rings twice, and the director, Gillies MacKinnon, hops out of his director's chair and scoots off down the hallway for a directorial word in Wilby's ear before they try the whole thing again."I watch it all on a monitor now, as we're shooting it," he tells me between takes, as the oil lamps are being topped up and Make-up are re- applying the bags under Wilby's eyes. He peers along the now empty passage, murmurs the word "madness" to himself, and retreats "Cut", somebody bawls. At the far end of the corridor, like a man at the wrong end of a telescope, Sassoon emerges from his room, dressed in slippers, flannelette pyjamas and a dressing gown. Another man opens his bedroom door, sees the ghostly apparition and scurries back to his bed.

A perplexed and sorry- looking patient shuffles out of his room, wrapped in a white sheet, and tries the door of the bathroom. Reaching one end and turning around to look back along the diminishing, smoky corridor, the effect is undeniable It's a trench The bell rings Silence It's the middle of the night A door opens. I step through one of the false doorways, and I'm halfway along a dingy, carpeted corridor, lit by oil lamps, with windows that look out vaguely towards backcloths of formal gardens or Scottish hills. At first sight, the artificial Craiglockhart looks something like a cross between a miniaturised Brazilian shanty-town made of pressed sawdust, and some strange industrial-size object boxed up ready for shipment.But that's only from the outside. At least, that's how it looks from the top of the fire escape, and eventually (after two rings on the bell) I'm escorted to the ground floor and through a heavy, grease-stained curtain, to get a slice of the action.The Wilfred Owen Society was not impressed, apparently, when it was announced that the actual Craiglockhart Hospital in Edinburgh - still up and running - would not be used for filming for practical reasons.

Instead, its atmosphere and interiors would be recreated in a dilapidated industrial shed in Glasgow. Around it, a legion of caddies and flunkies and gophers, all in regulation puffa jackets and Timberland boots, all clomping about with clip-boards and walkie-talkies and rolls of gaffer-tape for bracelets. A bell rings in the yard and everyone stops dead in their tracks, like musical statues. Then the bell sounds twice, and everyone starts scurrying around again. Everyone looks very, very chuffed.I'm always impressed by the number of people credited at the end of a feature film, but hadn't anticipated that everyone involved in Regeneration would need to be on location at the same time, from some of the film's financial backers to the best boy's third apprentice, making the set like a community of ants or a beehive Somewhere in the middle is a central chamber. Am I supposed to provide some talismanic presence, a kind of poet's blessing of the project during these last days of filming? Either way, everyone's extremely welcoming, and the smiling faces looking up from their computer screens and editing desks all tell the story of a good thing coming to successful conclusion. My father had more than one wife and rarely visited our home.