Classical music / Joanna MacGregor Queen Elizabeth Hall, London

Much revered, much recorded but only rarely given in concert, Bach's last work, The Art of Fugue, yields most to those who listen hardest. Bach himself offered few clues as to how - or even if - it should be performed, so any prospective interpreter needs to consider such crucial issues as style, playing order, presentation and instrumentation. Solo keyboard performances are particularly problematic in that a lack of imagination, not to say vitality, threatens to minimise essential contrasts between individual movements.

Not, however, with Joanna MacGregor, whose QEH performance on Sunday afternoon, like her new Collins Classics CDs, made for urgent reportage of some extraordinarily far-reaching counterpoint. MacGregor arrived on stage, slim, black-clad and armed with a large, spaciously printed score, while an overhead screen bore witness to her impressive finger velocity. After a slightly nervous first Contrapunctus, she settled to dazzle us with a reading that was so varied, dramatic and thoughtfully structured that matters of "authenticity" (piano versus harpsichord, organ or whatever) ceased to be an issue.

Contrapunctus No 2 paraded jazzy syncopations, then, beyond the excited closing bars of No 4, MacGregor paused for breath: she placed her hands on her lap, took a couple of seconds' rest and moved on - revelling in the ecstatic progress of, say, No 6 or the 10 minutes of chromatically shaded rapture that make up No 11. The most courageous aspect of her performance, however, was her decision to play virtually the entire work in the first half of the concert, reserving just the two-keyboard No 13 and its "mirrored" variation for the beginning of the second.

It was here that electrical gremlins began to intrude - most conspicuously via a mounted monitor that refused to function and weak play-back levels for the pre-recorded tracks that would, we had hoped, project MacGregor's "second self" into the hall. She was obviously confused, and not a little impatient (it later transpired that the real culprit was a rogue loose connection). "Andy, monitor please!" she shouted, looking towards the large console that dominated the middle stalls. Then, without further ado, she launched straight into three Canons by Conlon Nancarrow - a crowd of fidgety voices set at different tempos and calling for extraordinary agility and mental discipline. Next came MacGregor's own arrangements of three of Nancarrow's Studies for Player Piano (also included on the Collins CD) where, in addition to the live contribution, we heard - and watched - two, sometimes three, extra tracks: four Joanna MacGregors grappling with spidery counterpoint, cross-talking rhythmic patterns and echoes of blues, ragtime and boogie.

It was all very different to the clipped, ghostly mechanisms of the piano-roll originals. Alas, the monitor still failed and the pre-recorded pianos cowered in the distance. Resilient and resigned, MacGregor laughed, "I'm doing all this by sight!" And then, hey presto! the monitor suddenly sprang to life, and the other pianos with it. The hall was awash with laughter. Once modified, the technical formula worked a treat and the concert climaxed to a maniacal orgy of complex rhythms.

ROBERT COWAN