The Los Angeleno writer and director Stephen Sachs has written a play about authenticity. And yet it lacks any, even though it is based on a true incident. Quite how or why it attracted the infinitely watchable Kathleen Turner back to the London stage is hard to say. She plays Maude, a hard-drinking, splendidly foul-mouthed former barmaid whose home - a trailer in California - is furnished with junk picked up for a few bucks.

Her belongings include a painting which may or may not be a Jackson Pollock. Ian McDiarmid is Lionel, the British-born, public school educated art expert who has travelled from New York by private jet, and then by chauffeur-driven limo, to Maude's incredibly modest home to pass judgment on the painting's authenticity. And because he is a disdainful snob, on Maude, too.

Although most of Polly Teale's production follows on from Lionel's verdict, it would be wrong to loosen what tension it generates by revealing it here. But it doesn't much matter. It's that, even though we only see the back of it, it's not the painting that comes across as fake but the play.

Sachs asks us to believe that such broadly drawn characters as an art expert who makes Brian Sewell look like Del Boy and the kind of trailer trash dame that made Jerry Springer a good living would end up downing shots of bourbon as if they were old drinking buddies. Or that in the relatively few minutes after his arrival, the reserved and formal Lionel would end up on his knees passionately extolling the virtues of Picasso's Les Demoiselles d'Avignon before flailing around on the floor like an epileptic seal as he describes Pollock's paint-flinging technique. Really?

I don't mind the predictability of two entirely opposite characters finding common ground through each other's humanity. But it has to be done well. "Be a person," says Maude, encouraging Lionel to loosen up - one of several good one-liners in the dialogue. But they are spoken by characters whose behaviour is almost impossible to believe. Part of the problem is that the action unfolds over real time - just 80 uninterrupted minutes - and all the while the unseen chauffeur waits outside.

Turner does convince us that Maude and her homespun wisdom learned from the school of hard knocks is the real thing. Not so McDiarmid. Though I also blame Sachs for that. His version of a British public school prig is a charmless cipher created to embody snobbery. Though in my experience, whatever you think of the public school system, the one thing its former pupils invariably have in droves is charm. But then Sachs, you get the feeling, is kind of faking it on that score.