"Why are there so many doors? Was this house designed by a lunatic?" asks Dr Rance, the psychiatrist who comes to carry out a government inspection of the private clinic where the proceedings take place. Characters in a farce don't normally draw attention to the weird number of exit points needed for all that dashing in and out. Nor does the authority figure usually turn out to be the maddest person on stage.
But then Orton's piece is not so much a farce as an ingenious anti-farce - a gleefully anarchic mismatch between the conventions of the genre (which rely on a need to keep up an appearance of respectability) and the new sexual permissiveness of "letting it all hang out". In order to conceal the prospective secretary that he has just persuaded to strip, Dr Prentice (Jonathan Coy) embarks on a frantic cover-up that unleashes mayhem: transvestism, shootings, druggings and strait-jacketings. Why, though, does he bother, given that his wife (flamboyantly played by Belinda Lang) is a nymphomaniac, and the government inspector is a basket-case?
You can appreciate why Orton got up to this strange game of going through the motions of conventional farce as a way of demonstrating the breakdown of the rigid categories that propped up the form. "I must be a boy, I like girls" protests the desperate secretary (Joanna Page), when masquerading as a hotel page. "I can't quite follow the reasoning there," comes the reply. In a piece where the two psychiatrists wind up trying to certify the other as mad, Orton relentlessly exposes the dubiousness of the authority that decides the division between normal and abnormal, sane and insane, the straight and the sexually outlawed.
But for these revelations to be truly elating, there would have to be entrenched hypocrisy in the social microcosm he depicts. That's not the case, though. Malcolm Sinclair is very funny as Rance: quietly bonkers, like a dangerous zealot in the disguise of a stiffly upright bank manager. A paragon of propriety would not, however, let slip his own non-establishment inclinations, as when he declares that "Boys cannot be put in the club. That's half their charm".
To ensure that the wordy wit does not slow things down, the skilful cast pull off a feat that must be equivalent to racing around while balancing stacks of china. But the production, while entertaining, occasionally plays too safe. Instead of having only a policeman's helmet to conceal his manhood, X's blackmailing bellhop is allowed the insurance policy of Y-fronts. Which rather takes the arse from the farce.
To 20 August (020-7722 9301)