Forget good clean fun: Joe Orton’s What the Butler Saw is a hilarious, down and dirty farce of cross people and cross dressing in a lunatic asylum masquerading as a private clinic, where a job interview leads to a thwarted seduction and a knock-on series of catastrophes.

But as Sean Foley’s energetic production, very well designed by Alice Power, is at many pains to point out, the real joy of this 1969 classic resides in its jewelled dialogue, which is as witty and aphoristic as Oscar Wilde’s, and pushed to the limits of indecency.

As in Wilde, the humour lies in the rhythm of extreme antithesis, surprise vocabulary and an air of drastic finality. The temptation, to which the cast mostly succumb, is to confuse thumping authority with a sly and undercutting wit, so that the hectic physical action - the slamming of doors and thrusting of underwear into flower vases - is matched with a similar stress and vocal emphasis.

This makes the experience of sitting through the play slightly exhausting, but the ship stays afloat thanks to the brilliant comic contortions of Tim McInnerny as Dr Prentice and Samantha Bond as his braying, dipsomaniac, dissatisfied wife, a sort of ruined Coral Browne (who played the role in the first production); she exchanges her fur coat (covering a black negligee) for the floral dress of Georgia Moffett’s cute but sexually innocent interviewee, Geraldine Barclay, without even noticing.

Barclay in turn finds herself kitted out in the costume of the bell boy from the Station Hotel where most of the play’s marital and sexual confusions can be traced to a Boris Becker-style encounter in a linen cupboard. Director of the madhouse operations is the visiting inspector, Dr Rance, whom the comedian Omid Djalili plays with a ferocious, but very limited, barking bullishness.

Djalili’s technique of biting off phrases and spitting them out undermines the fluent brilliance of his diagnostic speeches and ludicrous conclusions. But there is so much to enjoy in every line of this play that the mechanics of the farce - which are perfectly aligned - can sometimes be taken for granted.

Nick Hendrix is too drag-queeny as the bell boy in a dress, but he and Jason Thorpe as the dull policeman sucked into a vortex of orgiastic mayhem play funny and straight where it matters most. And all evidence of moral turpitude is trumped by McInnerny’s delicious delivery, hair flopping manically, of the suddenly topical line, "There is nothing furtive in my relationship with the editor of the Guardian".