All too often, the compressed, feverish atmosphere of the Edinburgh festival inspires not criticism but hyperbole. Shows transfer to London and wither in the resulting glare of anticipation. For the four twentysomethings behind the Garth Marenghi horror spoofs, playing in London for the first time, the pressure is even greater. Their debut Edinburgh show, 2000's Fright Knights, was a word-of-mouth hit that was nominated for the Perrier award. Their second, Netherhead, won it. As Garth himself might say, the weight of expectation hangs like an extremely heavy albatross around their necks.
So it's all the more thrilling to see how the show thrives outside the Edinburgh hothouse. The trio on stage - Matthew Holness as Garth, Richard Ayoade as his editor Dean Learner, and Alice Lowe as the statuesque actor Alice Lowe - exude confidence. Their characters make mistakes, fluff entrances, undermine each other and deliver the daftest of dialogue, all with an unembarrassed air of high seriousness that renders things even more absurd. Director Paul King, meanwhile, keeps the action fairly taut and the production values winningly incompetent. Along with the atrocious acting, the props come straight from the 1950s sci-fi B-movie school of design: papier-mché birds fly at the ends of fat black sticks; an ancient Egyptian stone tablet is light as a leaf.
The Garth crew aren't the only company subverting amateurishness to make it a virtue, but their show feels singular in its attention to detail. Netherhead is a dramatisation of a Marenghi book about a man who unwittingly inhabits the plot of a horror novel, allowing Garth to make several pointed remarks about the excellence of the genre. He is the most agonisingly, amusingly literal of writers: his descriptive powers are limited to the line, "He walked via foot to his car", while a ghost announces, "I'm invisible." The bathos is delicious, nowhere more so than in Netherhead's opening sentence: "Romford was the cruellest city" - an echo of TS Eliot that perfectly captures Garth's dauntless ambition.
Every gesture in these rigid performances holds a laugh. Garth, in a fit of rage, hurls a table about the stage, but takes care to put it back in its original position. Lowe is perhaps the most impressive, because she is the most restrained. Her eyes locked on the audience, she speaks each line as if it conceals a profound message. If it does, Garth will be sure to explain it: "There's nothing worse than a play that leaves you with questions," he declares. The only question Netherhead raises is, where will the quartet go from here?
Until November 17. Box office: 020-7478 0100.
