Neighbourhood Watch is a great play. I thought I'd got the measure of Alan Ayckbourn, as a sort of Michael Frayn-lite; his plays characterised by a rich premise, undemanding execution and a fizzling-out at the end. I was clearly wrong, which just goes to show you should never underestimate a national treasure. This, his 75th(!) play, has a much messier set of ideas to explore and explores them much better. All without losing the froth.

The play centres on Hilda and Martin, a nice respectable brother and sister who move into a nice respectable area. Their efforts to form a well-organised neighbourhood watch scheme to defend their community from the local estate get a little overenthusiastic, and they begin to find the people they're locked in with are as bad as the people they're keeping out.

The ensemble cast weave in and out of each other's lives, suitably claustrophobically. We focus entirely on the Watch team - the estate dwellers are referred to but never actually appear to have their own say. All the actors are strong, though some parts are more three-dimensional than others. Most characters flirt with cliche, but largely come out on the right side.

I can't think of many contemporary plays which give Christians an even hearing. While Hilda and Martin's faith isn't exactly endorsed, it isn't panned either, Ayckbourn standing up squarely to the Dawkins-dismissive-atheist movement. Here religion is just thrown into the mix of things humans get right and wrong - along with gender roles, justice and protection, attitudes to the police, control, love and controlling love. It's as messy and interesting as life.

Some theatre leaves you uplifted; united in a shared bond with your fellow watchers. This play is so dark that the audience departed warily, suspicously watching each other, and hurrying away without stopping for a post-curtain drink. I never thought I'd see Ayckbourn do that.