THE programme for Ben Elton's rumbustious comic tale of infertility is presented in a medicinal bag by ushers in doctor's coats. Inside is a sperm specimen receptacle, on which is printed the result of a sample: 30 per cent sluggish, 41 per cent swimming in the wrong direction and 90 per cent useless.

Those figures are referring to the sperm count of one Sam Bell, the lead character, but they could equally be a comment on Elton's joke count in the first half, when Laurence Boswell's stage premiere of Elton's novel is not so much a play as a series of over-excited sketches performed by caricatures, and all the gags could just as easily come out of the motormouth of Elton himself.

For those struggling to make babies, infertility is a painful matter, rather than a cause for laughter, and a series of "unspeakably rude" jokes will not alleviate that sadness. This is not Doctor In The House or Carry On territory, and not until the last scene before the interval do professional London couple Sam (Duncan Bell) and wife Lucy (Geraldine Alexander) talk seriously or at any length, and even then Boswell's jaunty production spoils the moment with a door-slamming mime gag.

Sam is a jaded commissioner of comedy for BBC TV, Lucy has an uninspiring assistant post at Spotlight, the theatrical agents. Neither strikes home as a fountain of metropolitan wit and yet Elton - or rather Boswell's adaptation of Elton's book - has them speaking a series of short, sharp Eltonisms. "My womb is a prune," she thinks aloud. Doing likewise, he says: "Lucy is looking saucier than the condiments shelf at Sainsbury's." The words don't go with the characters, and nor do some of the more crude comments: perhaps it doesn't help that Geraldine Alexander looks like sweet and innocent Julie Andrews and Duncan Bell like one of Richard Curtis's buttoned-up and easily-shocked characters in Four Weddings or Notting Hill.

The play does settle down in the second half, when there is a better balance between the humour overload and the human story within, and the two leads become more credible. Meanwhile, the supporting cameos take on meatier life, especially William Tapley's thespian bounder Carl Phipps and Duncan Marwick's Irvine Welsh-spoofing film director Ewan Proclaimer.

Elton's best moments here are his digs at the modern, modish BBC, and a brief send-up of Oasis in a fusillade of swearing brings a smile.

Overall, however, Elton's technique is still to use a bludgeon rather than a scalpel, and his brash writing lacks sensitivity or warmth. The poster could not be more apt: with Inconceivable he has moved from rants to pants.