Dark comedy is a tough genre. To laugh, but to feel pain, to start, but still sympathise, is a tough balancing act. When you add that Cosi, which opened this week at the White Bear Theatre in Kennington, is not only a dark comedy, but is set in a mental asylum, well, when you settle down into the intimate space of the back-of-the-pub venue, it has to be with some trepidation.

But after a slightly slow expository first 20 minutes, it becomes clear that you are in the hands of a master dramatist. And that's a fact, for Louis Nowra is one of Australia's best-known playwrights, and the skill and the experience shows here in the balance of laughter and pain.

And the language is a delight - no surprise to this Australian-born reviewer, who recalls studying a play at high-school in which parliamentarians were called "scrufulous sheep", but there were shocked goggles in the London audience at lines such as "He's as testy as a ram wanting to get into the ewe paddock", but by the time we got to "you know what culture is to most Australians - what grows on stale cheddar" they were right in the swing.

But Nowra doesn't need specifically Australian references to have fun with language: a particular focus is the problems the transsexual inmate Ruth (Neil Summerville) has with illusion and reality. As she explains: "I can deal with things being an illusion or reality but not at the same time."

There's plenty of illusion in this tale, however, at least in the minds of the characters, as the inmates of an early 70s Australian asylum meet the young director Lewis (Matthew Burton) who's been employed to help them produce a play. Some cheerful simple comedy would seem in order, although Lewis has his heart set on Brecht, but one of the inmates, Roy, is determined to fulfil his life's ambition of starring in a production of Mozart's Cosi Fan Tutti, despite the fact that not one of the potential cast knows either a word of Italian, or anything about opera.