The Gogmagogs are the group of classical string players who turn music into physical theatre. Trained since 1995 by the opera and theatre director Lucy Bailey, they home in on the body language and emotional demonstrativeness that most musicians can't help showing when they perform, and shape it into a deliberate expressive force. Musicians wrap themselves around a cello as if it were their soul, or drag around a double-bass like so much psychological baggage. The act sounds as pretentious as their name, with its Cambridge allusions, but it packs a direct punch and has always gone down a treat.

Their current show, workshopped last year at BAC and now on its premiere run over the river, tries something new – they invent their own music, instead of adapting or commissioning from composers. It pushes the balance several degrees in the direction of theatre. Troy Town puts its cast into the shabby labyrinth of a prison, where they get lost and find themselves in what would be a take on A Midsummer Night's Dream if it didn't become the legend of Theseus and the Minotaur as it went on – except that here Theseus is the bad guy fighting for a woman who doesn't want him. Then it all turns out anyway to be a Big Brother exercise in the guise of a computer game.

Confused? You would be, without the information from the programme. It has some affecting and entertaining scenes. The instruments make props for helping to stylise sexual encounters and fights. Bailey can point up the futility of a male-rivalry show by making it subversively funny, though the climactic fatal encounter is done with slow, stylised power. There is an affecting love duet between cello and bass, and a comic phallic dance. But there is too much content and reference packed into the play's 70 minutes, and the short scenes rarely gel into a cohesive sequence as the characters rush around.

Part of the problem is the music. It's a liberating ploy to draw the sounds from the same source as the action, and I never thought in advance that I'd miss the guiding hand of a composer. This work-out of traditional Cretan music mixed with the players' own ideas certainly has its moments, especially from the droll bassist, Lucy Shaw, and from the lyrical lovers, and in the way that the offstage cast keep on playing like a Greek chorus in music. But the cumulative effect is short-winded and bland, relying on stock repeating devices, and it is in need of themes that can characterise people rather than generalised situations.

It doesn't help that the cast keeps resorting to words, as though they aren't confident that the music will put across the full meaning. Yet the show still works best when the cast's expressive skills, physical and musical, are left to speak for themselves. They don't act with their voices anything like as well as with their bodies, and they make the text even more bathetic than it already is.

Troy Town feels like a transitional work. There is an air of promise that the show after this will break through to the group's next level of communication. But as a show, this is one for the Gogmagogs's existing fans.

To 10 Nov (020-8237 1111)