Anyone whose education extended to a place in the school orchestra will know the drill. Sit up. Don't tap your feet. No grins or grimaces. Eyes on the score. At conservatoire level, too, instrumentalists have dinned into them a decorum that forbids all muscular exertion beyond what is technically necessary. Give listeners anything to look at, the thinking seems to be, and they won't be all ears.

Tosh, says the theatre and opera director Lucy Bailey. Dismayed by classical concerts (of contemporary music especially) in which the players seemed aloof and detached from their material, she wondered why audiences were regularly asked to watch something so visually unengaging. And why such music was rarely fun.

Try saying "the gogmagogs" with Radio 3 sobriety and you know that creating a good time is a priority for the group of young string players that Bailey now directs. At their debut a few years ago they took a mallet to classical preciousness: swapping instruments mid-melody, wearing wacky clothes, dabbling in pop choreography. Their second show, the gogmagogs gigagain, which premiered as part of the City of London Festival last week, takes a more thorough and sometimes subtler approach to theatricalising the music. Freed of the physical impedimenta of scores, stands and chairs, and the furrowed brows that come with them, there seems no limit to their inventiveness.

Yet it still comes as a shock to see a jumble of bodies and instruments (heavens, the insurance!) on the floor. More so when the inanimate heap twitches into the taut Palestinian rhythms of Said Murad's Tasaod ("Getting Up"), seven bodies and seven instruments rising and swaying like a single reed in the desert wind. From eastern Europe, Haukur Tomasson's piece Mannemot ("People Gathering") offers a vivid moving-picture of a protest march - its confident advance, unplanned incidents, violent confrontation and nervy retreat.

My favourite was the gogs' treatment of Mike Westbrook's Cable Street Blues, the players goggling at an imaginary cinema screen, registering the peaks and troughs and nerve-jangling horrors of the movie through finely graded facial and bodily contortions to brilliant comic effect. And all the while bowing their way through Westbrook's tight-knit jazz counterpoint with stylish aplomb.

Just once or twice the play-acting descends into whimsy. The hissing, disembodied faces of Jane Gardner's Night Choir were effective first time round, tiresome at length. Her companion piece Monsters (with instruments where the players' heads should be) fell flat both as music and as theatre, though you could not but marvel at the technique required to finger a cello on the horizontal behind one's ear.

The essence of the gogs' success is that they are, first and foremost, a stunningly good string septet. Intonation and ensemble are as taut and secure when double bass and cellos are 10 feet apart as if they were conventionally seated. What Bailey has done is refuse to accept that specialists of this calibre cannot legitimately extend their talents into other performing arts - a blurring of boundaries that is no less courageous for being very much of the moment. They do not possess dancers' bodies, but still move with verve and conviction. My perception was that those with the larger instruments slung round their necks (on a special harness) seemed marginally happier than the fiddlers, who were more physically exposed.

All eight pieces of music in this set are new, and specially written or adapted for the group. Bravo for the sponsors' vision that made this possible. This is a novel hybrid that compromises none of its elements. In fact, it was cellist Matt Sharp's solo singing (in John Tavener's heady Greek Orthodox lament) that, for me, raised the evening to a pitch of spiritual feverishness rarely encountered in either concert hall or theatre.

Bridewell Theatre, EC4 (0171 638 8891), to Sat.