Three years ago, Béla Fleck extended his already considerable musical range with an intriguing album, Throw Down Your Heart: Africa Sessions, the result of the virtuoso banjo-player's travels across the continent, during which he recorded with the Malian diva Oumou Sangaré. He has taken that collaboration further with a concert that provided further proof of Fleck's instrumental genius, but never the subtle experimental collaboration that might have been expected.

It started with Fleck sitting alone on stage. In jeans and a short-sleeved shirt, he looked more like a genial traveller than the winner of 18 Grammys. His remarkable solo set included excerpts from a banjo concerto "that I just wrote" and a thoughtful, then a rapid-fire progressive bluegrass workout in honour of his early banjo hero Earl Scruggs.

The stately Sangaré then joined him on stage, accompanied by one singer, a rousing kamalengoni harp, and less effective, full-tilt electric bass and drums. She was on powerful form, and Fleck now became part of her backing band, adding gutsy amplified banjo riffs and the occasional solo as she switched from a song from Timbuktu, which allowed her to comment on the tragic upheavals in northern Mali and give an unwise invitation to the audience to dance on stage.

The best song, by far, was the exquisite and delicate Djorolen, for which she was backed only by Fleck's thoughtful and delicate banjo work and by muted hand percussion. Baaba Maal, who had introduced the show, reappeared to announce "this was one of the most beautiful songs I ever heard", and break into a praise song to Sangaré. But there the acoustic fusion experiments stopped. The band returned for a furious jazz-rock workout, and stomping treatments of such favourite Sangaré songs as Seya. The audience were delighted, but an opportunity had been missed.