Bach wrote these six trio sonatas as teaching material for his eldest son, Wilhelm Friedemann. They're a tough challenge, music normally conceived for four players - two upper parts, bass and continuo - but played by one organist on two contrasting manuals and pedals. Reversing the concept by transferring the music back to violins, cello and harpsichord is an imaginative move; other ensembles have tried it successfully with different treble instruments.

The Brook Street Band plays stylishly, with well-nigh impeccable intonation and tremendous verve - some of their tempos would push a solitary organist to the limit. But where Bach's contrasting organ manuals deftly weave their differently-coloured lines, here two identical violins turn dialogues and imitations into mere repetitions. The problem's exemplified by the opening movement of Sonata BWV528: this is borrowed from Cantata 76, but there Bach specifies vividly contrasting oboe d'amore and viola da gamba. Perhaps wider stereo separation would have helped, and violins could have contrasted tone, dynamics, and articulations more distinctively. In live performance where the eye can direct the ear to spot who's playing what, or by following Bach's organ score, the vitality is riveting and the counterpoint enthralling. Delightful. Performance * * * * / Recording * * * *