First Night: White witches' musical rite

I DON'T want to be rude about the Mediaeval Baebes, primarily because I value my health and everyone of these girls is, I am assured, a witch. They say they are white witches, and it is all done in the best possible taste. I also don't want to be rude because the Baebes don't deserve it.

Twelve feisty chicks, they sing songs from the swingin' 1360s while trussed up in see-through drapes. But this kind of schtick isn't new - we already have Vanessa-Mae and a host of PVC-clad opera singers. Signed to the Spice Girls' label Virgin they would be easy to dismiss as a marketing ploy; they include among their number a stripper, a go-go dancer, a welder and a systems analyst, and come out with eyewash such as "I like wearing ivy and nothing else", and "I design spaceships. That's what space pixies do". Add to this the fact that founder member, Katherine Blake, also formed Miranda Sex Garden and you are definitely working with one hand tied behind your back.

But this could all be tongue in cheek, because the Baebes have brains, and they do what they do quite remarkably. When they arrive on the venue's cramped stage - where the microphones are growing plastic ivy - their very presence seems to fill the air with something oddly arcane. They are like novices let loose from a convent and given a crate of beer, though they have dumped the virginal white they used to wear for vampy, red robes; 12 unpredictable Ophelias gone to the bad, bohemian and startlingly good- looking.

Then they open their mouths and begin to resonate, and it is truly scary. "Kinderly" is a wild roundel with a vicious, booming bodhran that chills the blood; then, like an echo of Palestrina's unwordly, unaccompanied vocal beauty, "Now Springes the Spray" fills the room with its Chaucerian words.

It is hard to believe only one of the group has musical training: breathing and pronunciation are perfect. And eerie, too. A plainsong tale of lovesickness so deep "That slepen I ne may" - that I cannot sleep - it holds everyone in a hypnotic trance and the hairs stand up shiveringly on your neck. "Adam lay I bounden" brings in - gradually - flute, tambourine and a drum that sets up a slow, unearthly rhythm, as threatening as anything from Carmina Burana.

Occasionally, the trance breaks or the tone has an awkward nasal glitch and what they are doing seems pompous and silly - but not that often. Toward the end, they give us "Summerisle", the 13th-century number to which Edward Woodward is burned alive in the cult film, The Wicker Man. By now, they look both relaxed and possessed, sweeping the audience with their piercing gaze. Coming soon to a sacrificial rite near you.