The Wonder of Sex, NT Lyttleton, London

Sex – all done in the best possible taste

It's hard to think of two people less likely to be helpful as sex counsellors than the National Theatre of Brent's cuff-tugging luvvie Desmond Olivier Dingle (Patrick Barlow) and his verbally challenged, toupee-wearing sidekick Raymond Box (John Ramm). Even the basics – what to put where and when and why – are bound, you'd have thought, to be quite beyond them. Trying out positions under their tutelage would surely be the shortest route to intensive care or Strangeways.

So, an evening where our duo don professional white coats in a life-size replica of a state-of-the-art therapy unit and take us on a guided tour of "The Wonder of Sex" is calculated to leave you feeling more screwed up about the subject than when you arrived. The show begins hilariously with a re-enactment of the origins of that primary fatal disorder, the Oedipus Complex ("Now we've torn it," tuts Raymond's rather suburban Jocasta when she realises she's been schtupped by her son), and continues on the same level with Freud's First Case, in which the great psychiatrist gives a session to Madame Arcati (Raymond wearing black bonnet), who is clamped like someone on the Big Dipper in an alarmingly high-tech counselling couch.

Given Raymond's terminally shaky grip on the English language, which seems to start disintegrating when it sees him coming, you'd be hard put to distinguish in his discourse a Freudian slip from all the other kinds. "I am a highly sexually compliant and repulsive woman," he rambles, "locked up in the hidebound stays of Victorian England, unable to express her penthouse sexual dreaming – what will I do?"

All the familiar landmarks of a National Theatre of Brent show are present and incorrect. There's Desmond's repeated mean thwarting of Raymond's desperate desire to do his star-turn Rasputin impersonation. There's the delightful audience-participation section, in which the house is divided into "three halves" – as Russian peasants, Russian townsfolk and "dreaded Trotskyites" – each shouting a "historically authenticated actual comment" at the storming of the Winter Palace.

But the show has none of those mysteriously touching moments you got in their wonderful Charles and Diana saga, Love Upon the Throne or their Messiah, where the bungling comedy of incompetence highlighted an unexpected humanity in the subject, and where the stilted banality of the dialogue occasionally brought a lump to the throat.

And while there are a couple of sweat-inducing sequences, such as when unmarried couples in the audience are quizzed as to whether they have torrid designs on the person they are sitting with, "a shocking yet tasteful exposé" of sex begins to feel like a contradiction in terms.

There's a problem, too, with the venue. The National Theatre of Brent has graduated to the National Theatre of Great Britain, colonising the historic "Littlewoods" theatre, with complicated computerised equipment responding (or failing to respond) at a flick of Desmond's "remote". The duo get a certain amount of comic mileage from the piquancy of this pairing. But it's rather as though the Pooters from Diary of a Nobody had been suddenly elected Lord Mayor and Lady and given the run of County Hall. It's the self-deceived nature of Desmond's portentous grandeur (with that constantly slippin' plummy accent) that makes him funny. Put him where he aspires to be, and the joke kind of cancels itself out.

Still, if this is not premier cru Brent, it's miles more potable than most Christmas plonk. The man I heard on the way out saying, "Yeah, very funny, but they shouldn't be here", got it in one.

To 16 February (020-7452 3000)