Dr Wicksteed seems to have vanished, but Mrs Swabb has seen him standing on Hove pier. "I fear the worst," cries Mrs Wicksteed. "Death?" asks Sir Percy Shorter. "Sex," she corrects him.

This near-equation of the two great levellers is what drives Habeas Corpus along. Alan Bennett himself says in a programme note (for Sam Mendes's production at the Donmar Warehouse) that sex is what his 1973 farce is about; but that's to sell it short. It's a frenetic meditation on how we deal with our bodies - in particular on the mixture of feelings that the English nurse for their bodies: shame, revulsion, puzzlement, amusement, outright denial.

Except when the lovely Felicity Rumpers enters, or repressed spinster Connie Wicksteed takes her new false breasts out for a test-drive, revulsion is uppermost: "Show me a human body and I will show you a cesspit," says Dr Wicksteed, the outwardly respectable GP in whose household the action takes place. When Mr Shanks, the fitter of false busts, discovers that Mrs Wicksteed's breasts, on which he has been lavishing his attentions, are the real thing, he recoils in horror: "Is there anywhere I can wash my hands?" There's too much touching these days, laments Lady Rumpers: "If I want to be touched I have people who love me who can touch me. Touching is what loved ones are for, because the loving takes the sting out of it."

Mendes catches the revulsion brilliantly, most powerfully through Jim Broadbent's Wicksteed. At first, he seems an odd choice for the part - a little too seedy and disreputable; but as the action progresses, you realise just how well he is suited to playing a middle-aged lecher for whom sex is the only escape from death ("He whose lust lasts, lasts longest" is the moral he pronounces at the end of the play). With his greasy, thinning hair and his lumpy sack of a body inside ill-fitting clothes, Wicksteed is clearly a prisoner of the flesh who's on the verge of going stir-crazy. At the end, the entire cast begins a slow Latin dance; but the lights go down and everybody else fades away, to leave Broadbent to perform his solo dance of death in an ungainly and ever more frantic hop and shuffle, until he finally seems too exhausted or despairing to carry on; for all the dark threads that run through the evening, that moment is startlingly saddening and frightening.

If Broadbent's performance takes you by surprise, Brenda Blethyn succeeds by doing precisely what you expect, only better. She plays Wicksteed's wife, Muriel, a middle-aged wardrobe of a woman who wonders where all the magic has gone, and nearly recaptures it under Mr Shanks's expert probing. You imagine that when he wrote the part Bennett had in mind precisely Blethyn's combination of quiveringly fragile respectability and scarcely repressed sensuousness, which explodes into an orgy of bumping and grinding, thrusting her bust in Mr Shanks's face, squealing with pleasure as he examines her.

There are places where Mendes's production goes astray - the beauteous Felicity's enthusiastic reception of overtures from Dr Wicksteed seems completely beside the point; and you wish they didn't play around with the lighting so much. You do wonder, though, just how far the faults lie in the play - there's a sense that Bennett hasn't quite got his material under control, both in the demands of the stagecraft (it's a minimalist farce with virtually no props, and that doesn't give the actors much to work with) and sometimes in the way that he co-opts other writers. The echoes of Betjeman, as in Wicksteed's wistful rhapsodising over "Busty girls in flowered scanties, easing off St Michael panties", carry precisely the right air of forlorn bourgeois sexuality; but at other points - in a brief reference to The Importance of Being Earnest, for example - the parody feels more like a cover for Bennett's uncertainty over how to modulate a scene. These are minor carps, though: Habeas Corpus remains a clever, very tragic comedy, and the production matches up to it.

No discomfort with the body in evidence in Claustrophobia. The Maly Theatre of St Petersburg has been touring its latest show around the country, finishing up last week at the Lyric Hammersmith. The fact that it's no longer playing is a weight off my conscience, since I honestly don't know whether I could recommend this bizarre, overlong piece: it runs over two hours without a break, and there isn't anything you could call a plot to keep it going. But the Maly's physical virtuosity is extraordinary - dancing, climbing walls, throwing themselves into the acting with a sheer expressiveness and intensity quite beyond anything I've seen come out of this country. In this portrait of a generation without conviction, trying to construct their own reasons for living, there are striking images; and one thing it isn't, with its spare, bright white set and its large, impatient cast. is claustrophobic. It's one of those things you probably ought to try once. One person I spoke to at Wednesday's performance was doing it for the second time, and that seems masochistic.

Soul and body again in Who Shall Be Happy ... ? at the Bush - Trevor Griffiths's account of the last days of Danton, imprisoned in the Palais de Luxembourg in April 1794. Danton is here, as everywhere else, presented as the human revolutionary - a sensualist as well as a politician, in flattering contrast to the sea-green incorruptible Robespierre. As he awaits the end of his trial and his inevitable execution, he tries to persuade his jailer, Henry, that he is not really Danton but an actor placed there as a decoy; meantime, he is trying hard to compose his last words. This is a play to see twice, if only to be sure you've got a handle on all the arguments; but for all its intellectual density, it doesn't quite take off as drama. There are good jokes ("Our lives are much like the theatre," the prisoner pronounces. "Indifferently written and scandalously short of rehearsal."); but there are also too many where the punchline is telegraphed long in advance. In Griffiths's own production, Stanley Townsend's central performance is a little too self-consciously big and charismatic to be winning. It's not a full meal, then; but it gives you plenty to chew over.

Theatre details: Going Out, page 14. Robert Butler is away.