Two Thousand Years, Cottesloe NT, London

It says a lot for the trust this generates that 16,000 folk have been only too thrilled too splash out on what was then a mystery, untitled product and that actors, left in the dark about everything but their own character, have been prepared to go through weeks of improvisation in conditions of secrecy that make the preparations for the Normandy landings look recklessly exhibitionist.

The one snag with this highly private approach is the risk of arousing a degree of expectation that only the unveiling of a major work could satisfy. That's not the ideal frame of mind for receiving the much-anticipated, but actually rather modest Two Thousand Years.

Set in a stripped-pine Cricklewood residence, it's a sympathetic, well-researched, amusing, patient - and also a touch too deliberate - piece about the tensions that develop within three generations of a secular Jewish family, when the 29- year-old son turns to religion.

You think that brooding Josh (Ben Caplan) is going to shoot up when he draws the curtains and rolls up his sleeves. But he's in fact binding his arm with the tefilen in preparation for prayer. The irony is that his liberal, Guardian-reading parents would almost prefer him to be a junkie. "It's unbelievable. It's like having a Muslim in the house," exclaims his left-leaning dentist father (likeable Allan Corduner) on discovering him in a skullcap. His nice mother (excellent Caroline Gruber) thinks, though, that it's adolescent of her husband to want a bacon-and-egg feast in protest.

The play proceeds to examine the assumptions of all the characters, who include an idealistic interpreter-sister (Alexis Zegerman), her deflatingly realistic expatriate Israeli boyfriend (Nitzan Sharron), and - best of all - the comically bolshie, fag-puffing grandfather (winningly played by John Burgess), once a kibbutznik - now dismayed at how Zionism has been hijacked by zealots.

When Josh asks the old man "What does it mean to be Jewish?", he jests that "It means visiting your family on a Sunday afternoon and finding yourself in a fucking war zone". Two Thousand Years is better at dramatising the tragicomedy of these emotional conflicts than it is at scripting the family's pretty contrived-sounding discussions about key political events in the past two years (from the depressing turn-out at the last general election to the removal of settlers in the Gaza Strip). But as it gradually conjures up a Britain where sensitive people have been left without a party to represent them, the piece makes scepticism about Josh's faith seem itself a form of intolerance. As the initially scathing grandfather concedes, he has at least found something that has meaning for him. A good, but slightly pedestrian play that lacks the layered richness of Leigh's finest stage and film drama.