Music Phil Johnson 20TH-CENTURY SAXOPHONES / SHEPPARD & HARLE The Barbican, London

When a doom-laden, neo-medievalist piece about the burdens of sin called Terror and Magnificence suddenly slips into the sand-between- the-toes shuffle of a samba for its penultimate movement, it's certain that something is breaking down, though whether it's musical barriers or simple good sense taking a tumble isn't immediately clear.

When the rather stiff composer, John Harle, shook a leg from within his curiously too-tight suit, and he and Sheppard tootled madly to the feel- good rhythm, it was a strange moment. It was made still stranger when the taped voice of an actor reading a 14th-century French lyric returned to boom out of the speakers, accompanied by the mad clatter of taped percussion. The quick-change of genres left the senses reeling but, fittingly for the programme's closer, it was something of a triumph and the most convincing performance thus far. The bridge between Harle's classicism and Sheppard's intuitive improvisation hadn't really been crossed all night, and at least Terror and Magnificence - based on poems by Guillame de Machaut - dared to be different; the Brazilian interlude got our feet tapping at last, too.

The partnership of Sheppard and Harle began after a concert in memory of Adolphe Sax, the inventor of their instrument, but the programme they devised for this project was less an attempt at summoning up the century of the saxophone than a familiar mix of favoured party-pieces, topped off by two special compositions, Harle's Terror and Sheppard's Trapeze. It was also, perhaps, partly a case of each player coveting the other's audience in a bit of inter-market penetration. With the promoters (and Sheppard's agents) Serious Speakout, following trends in new music, "jazz goes to the Philharmonic" was always on the cards some time soon.

When, after an opening from Harle, Sheppard took the stage and began a solo tenor-sax improvisation as a prelude to Carla Bley's gospel pastiche, The Lord Is Listening to Ya, all his many talents were immediately apparent. Sheppard has a beautiful, singing, tone and a natural, thinking-on-his- feet musicality, drawn at least partly (for he is self-taught) from his experience as a choirboy. His technique and his distinctive instrumental voice have improved to the point where he can sound truly masterful, but this hasn't answered the problem of what it is that he should play. Neither the extract from his and keyboard player Steve Lodder's music for the recent Arena series on Peter Sellers, nor the new composition Trapeze, were persuasive. Harle, by contrast, writes ambitiously and well, with a real sense of structure. He also plays the saxophone repertoire superbly, but, at least judging by the little we had to go on tonight, as an improviser he sounds rather formal, as tightly jacketed by his technique as he is by his suit. Though this may be the predictable, hackneyed response, the differences between jazz and the classics seemed even stronger than they were when George Russell wrote A Bird in Igor's Yard 50 years ago.