With main players including former Home Secretary David Blunkett and Kimberly Quinn (his former mistress and mother of his son), the Tory MP, TV pundit and magazine editor Boris Johnson and his journalist mistress Petronella Wyatt and former Leader of the Opposition Michael Howard, you might be forgiven for thinking that Who's the Daddy? is the latest in a series of verbatim dramas; this one re-playing the tabloid sex-and-lies scandals that unfolded in the offices of The Spectator in the summer of 2004 for public inspection.

In fact, it's anything but that. Toby Young and Lloyd Evans's new play, premiering at the King's Head, is billed as a "satirical fantasy". Far from placing its subjects under serious investigative scrutiny, it turns instead into a prurient but old-fashioned sex farce that is at once unseemly and slightly distasteful, if not ever truly disgraceful.

While the authors - the Spectator's own theatre critics - may have revived an all-but-extinct genre, their work simultaneously flatters and humiliates its real-life characters; flattered that their mostly ineffectual, narcissistic lives are being celebrated at all, but humiliated that they're all being shown as such grasping sexual predators (which may be a kind of flattery in this world, anyway). Actually, there's a human tragedy at the heart of this story - of how a high-flying cabinet minister was felled by falling in love with an unscrupulous social climber and found himself out of office as a result; and how not one but two more married men had affairs behind their wives' backs that destroyed their marriages. That is barely touched on here.

Though Young and Evans have constructed a plot of ever-spiralling chaos that puts its cast in and out of beds and cupboards amusingly enough (and director Tamara Harvey harnesses it with a sprightly production), few of the lines actually detonate with more than a smug smile. "I'm Rod Liddle, but that doesn't mean I've got a little rod" is just a representative sample from the level of schoolboy humour that prevails. As for topicality, a line like "What's it like having sex with a South American toyboy?" that gets the reply, "Ask Peter Mandelson" is hardly up-to-the-minute.

But a strong cast deserve plaudits for carrying on through the inner groans they must be suppressing. In particular, Sara Crowe as Johnson's mistress Petronella Wyatt is hilariously caricatured throughout, while Michelle Ryan also scores highly as an undercover journalist ("What, a journalist in The Spectator? Surely not!" exclaims Blunkett), and Claudia Shear (last seen in London in the Broadway import of her play Dirty Blonde) is so odious as Kimberly that you wonder what Blunkett ever saw in her. But then, as the show frequently makes fun of his blindness, he couldn't see her at all. And he's lucky that he can't see this play, either.

- Mark Shenton