Jonny Greenwood/London Sinfonietta, Royal Festival Hall, London

It was perhaps appropriate on the weekend that Doctor Who returned to British TV screens after a long absence that Radiohead's Jonny Greenwood should be championing the ondes martenot at the closing event of the South Bank's 2005 Ether festival. The instrument's previous claim to fame was mostly its use in sci-fi soundtracks, including the opening bars of the Star Trek theme.

It was perhaps appropriate on the weekend that Doctor Who returned to British TV screens after a long absence that Radiohead's Jonny Greenwood should be championing the ondes martenot at the closing event of the South Bank's 2005 Ether festival. The instrument's previous claim to fame was mostly its use in sci-fi soundtracks, including the opening bars of the Star Trek theme.

Somewhat perversely, given the obscurity of the instrument, the assembled seven ondes martenot were the common feature of the most accessible Western European pieces of the evening: Messiaen's "Oraison", Greenwood's own 2004 composition "smear" and, the climax of the event, two Radiohead songs, "Arpeggi" and "Where Bluebirds Fly", on both of which Greenwood played one of these instruments.

The last two pieces featured a typically but endearingly dishevelled Thom Yorke, whose Radiohead vocals often seem more like another instrument than simply a human voice, and this was certainly the case here. Despite the orchestral setting, "Arpeggi" was recognisable, instantly seductive Radiohead, as far as the musical construction and lyrics were concerned. Hands initially in pockets and then thrusting urgently at an invisible keyboard, Yorke sung of the ocean and our insignificance within its vastness. Winningly, he was greeted at the end of the track by an infectious thumbs-up from Greenwood at the back of the stage.

On the second of the Radiohead songs (originally a B-side to 2003's "There There"), he was joined by the Palestinian singer Lubna Salame, and the two combined to provide additional textures to the many already emanating from the ondes martenot ensemble and the full Sinfonietta, itself supplemented by members of the Orchestra of Nazareth. Earlier in the evening, the most audience-friendly pieces had been those performed by the members of the Nazareth Orchestra and Salame, its resident singer. Greenwood often cites the Arabic influences in his and Radiohead's music - on the Amnesiac album, for example - and the inclusion of these pieces emphasised that debt.

But this was not fundamentally an audience that was looking for audience-friendly material, and I suspect that Greenwood had never intended to supply it. He was leading us into areas that few typical Radiohead fans would go. The master of ceremonies had welcomed us at the start of the evening as "sonically brave", and we certainly had to be for the works selected by Greenwood from the oeuvres of Ligeti and Penderecki and especially the four brief pieces by Dutilleux.

We also heard the world premiere of Greenwood's composition "Piano for Children", which in its own way was just as challenging. After perhaps four minutes of strident but fairly conventional piano accompanied by the Sinfonietta's strings, the keyboard part then became discordant and disjointed, with beats on the bodies of the string instruments becoming the percussive glue that held the piece together. The Nursery Suite or even The Young Person's Guide to the Orchestra this wasn't, but presumably that wasn't the point.

The evening as a whole further demonstrated Greenwood's extraordinary talent for communicating his enthusiasm for pushing the boundaries of not just popular music but indeed all music. I was reminded of the recent description of Stephen Sondheim as having "avant-garde sensibility in a popular art form", and also of the interest and understanding engendered in me as a youth when I watched Sondheim's early collaborator, Leonard Bernstein, lecturing on TV about Stravinsky, whose music had seemed so obscure and arcane to me before then.

This may not be a mission in the conventional sense, but lead on, Jonny, lead on.