City Of London Festival

The sun sets on an evening of beauty in the Gherkin

Ascending the phallic "gherkin'' building in a high-speed lift, you feel rather like Woody Allen acting the role of a sperm in Everything You Wanted to Know about Sex.

The experience is only slightly more available than success as a sperm - like most of London's high-rise viewpoints, it doesn't often let the public on board. But over the years the City of London Festival has grown a reputation for persuading City institutions to open up their often spectacular premises as concert venues, and this year it was the turn of the Swiss Re erection to have its number called for the opening night.

It turned out to hold not much over a hundred people, many of them on this occasion familiar faces in London's classical music business. Still this was an evening of unexpected beauty. Whatever the building's origin and function it became part of a larger and different whole. The musical glow of Stockhausen's Stimmung was surrounded by astonishing sunset views in a hazy but cloudless sky. Quite apart from the landscape spreading from the Weald to the Chilterns, there were new perspectives of the Lloyds building and even the Tower. Birds flew slowly around the tip of the building, enjoying the thermal current. A light projectionist couldn't have done it better.

Only the rolled joints were missing - for Stimmung is pure 1968, the work that took its composer out of the old modernist world into his own brand of minimalism. The phase didn't last, but it left this wondrous monument, 80 minutes all built on one cord. Six singers sit around the circular table with a globe light at the centre, quietly humming into hand microphones, chanting syllables and exploring overtones.

Into this serene but highly charged soundscape they occasionally fling verbal volleys, the names of gods, and a series of steamy if sexist erotic poems of Stockhausen's own. Not much in the way of rhythm, but nobody's perfect.

Gregory Rose's group Singcircle are old hands at this piece. The group's name deliberately evokes the work itself, and they have sung it in German and English, at universities and The Proms, and on one memorable occasion after a major falling out with Stockhausen in rehearsal.

He may differ, but Singcircle's version remains to most listeners a definite event in its own right. Here again it came to life as the outside light changed and a sense of floating in the sky - usually an effect of the music in any case - had its literal reinforcement from beyond the building's glass canopy.

Honesty makes it necessary to report that the sound engineers' best efforts didn't always tame the natural resonance that the glass inevitably set off when the music grew louder. The Radio 3 relay later this summer may well sound more exact though the other dimension of the performance will have to be imagined.

For the visual counterpoint was one that even the works' creator, whatever the extremes of his imagination, can scarcely have conceived from the start. They will surely have to put it on permanently, once the word gets around.