On paper it sounds like a neat idea - the famed National Theatre of Brent appearing within what they call "the hallowed portals" of the real National Theatre. But it doesn't quite work. Since part of the joke is that this two-man company tackles epic subjects in tiny spaces it seems gratuitous to put it into the expansive Lyttelton.

I also feel that this new show - the pair's third on the ever-interesting topic of sex - takes them backwards rather than forwards. By that I mean that they have lately learned to balance satire with feeling - both in Love Upon the Throne, which dealt with the isolation of Princess Diana, and in last year's The New Messiah, where they showed that two squabbling theatrical incompetents could somehow stumble into moments of magic. It was as if Patrick Barlow, the company's prime mover and impersonator of the presumptuous Desmond Olivier Dingle, had realised that after 20 years it was impossible to go on doing variations on 1066 and All That.

Here, however, we are back to square one. The bullying Desmond and his hapless sidekick Raymond (John Ramm) aim to give us a clinical history of sex starting with the Oedipus complex and Freudian analysis. But far and away the best moment comes when, at Raymond's insistence that he play Rasputin, they drop the subject altogether and invite the audience to participate in re-creating the Russian Revolution. Stalls and circle compete in shouting incendiary chants and, against the backdrop of what Desmond insists on calling the Bournemouth Winter Gardens, the stage fills with volunteer Cossacks mounting cavalry charges.

After the interval, however, we are back with sex in the shape of Salome, Casanova and Lady Chatterley, and the joke quickly palls. The only interesting variation comes when the adenoidal Raymond, in the process of impersonating what he calls "the lusty gamekeeper David Mellors", develops a sudden physical crush on Desmond's bonneted Lady Chatterley. But all we are finally left with, in Martin Duncan's production, is a running gag at the expense of the National's technology. Armed with a hi-tech zapper, Desmond seeks to control sound levels, scenic trucks and a cybersex experiment only to find the machinery exploding in his face. But, by the time we get to an old Buster Keaton gag with a framed flat falling over the actors' heads, desperation has set in.

Barlow and Ramm remain highly endearing performers: the queeny boss and his bullied sidekick always in danger of losing his rag and his rug. And there are some choice moments in the first half. But when Desmond says, "You know where you are with bricks and mortar," he speaks the simple truth. He also makes one long to see the National Theatre of Brent return to doing epic shows in small spaces with a touch of unexpected emotion.

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