CLASSICAL Fretwork Wigmore Hall, London

The news that the counter-tenor Michael Chance had awoken voiceless on the day of Fretwork's 10th anniversary concert must have come as a rather unwelcome birthday present. The more so because Fretwork is not just an ace viol consort but a vibrant performing ensemble commissioning composers of today whose works are programmed alongside masterpieces of England's golden age. In no less than four contemporary works, Chance was to have been the vocal soloist, so finding a replacement for this repertoire at such short notice must have felt a tall order. But on Sunday at the Wigmore Hall, all hands (or friends) were on deck to make sure that Fretwork could justly celebrate their first 10 years, retaining virtually the entire programme, through the gallant intervention of Emma Kirkby and Deborah Miles-Johnson. The six-viol consort is one of the wonders of instrumental sound. So homogeneous in blend and so affecting in colour, this reedy group offers much to composers today. And not just composers brought up with the knowledge of England's great heritage.

Emma Kirkby, having sung with touching purity Orlando Gibbons's Go from my Window, switched to a purity of a different culture where vocal glissandi and whispered syllables breathed life into an eighth-century Chinese text. Tan Dun's A Sinking Love (1995) takes material from Purcell but in his use of tremolo, sudden loud notes, harmonics and koto sounding pizzicato, the different worlds are marvellously integrated. Like most chamber groups, Fretwork prefers to perform without a conductor, but in these circumstances of requiring a replacement singer it was fortunate that George Benjamin was on hand to steer his Upon Silence (1990). This is a remarkable work, a setting of Yeats's "Long-Legged Fly", where Benjamin has found a newly expressive sound world - rasping colours in rich dissonant chords, pizzicato and tremolo effects. And in his use of the voice in hesitant parlando style, the claustrophobic nature of the poem is decisively captured. Deborah Miles-Johnson was the very able soloist.

A far lusher harmonic palette was to be found in Gavin Bryars's purely instrumental In Nomine (1995). This is a small gem, the consort of viols providing both a richly sonorous sound and a reedy environment for Bryars's mournfully melancholic style with its throbbing repeated figures and eerie octave doublings.

With these recent works and assorted Fantasy, Fantasias and In Nomines by Byrd, Purcell, Jenkins and Lawes, Fretwork was in its element; their intonation is impeccable, their ensemble playing immaculate, their scholarship fastidious. But just when perfection seemed in the offing, an extraordinary miscalculation edged into the proceedings. What can the motivation be for working with a croaky, wobbly, out-of-tune pop star? Elvis Costello undoubtedly has his own following for his own type of work, but singing John Dowland is not his bag. And just what has this to do with Fretwork?

How is it that grown men (and women) cease artistic judgement when "cross- over" rears its ugly head? A marvellous occasion, strangely blighted.