Thom Yorke sings the new “Arpeggi” with the London Sinfonietta

Thom Yorke sang a new song, “Arpeggi”, for the first time at the Royal Festival Hall on Sunday night, reading the words from lyric sheets. Some of its lines, about the deepest ocean and the bottom of the sea, had already appeared on radiohead.com. Ed O'Brien, Nigel Godrich and Beck were in the hall.

Yorke's two songs closed Jonny Greenwood's programme with the London Sinfonietta, the two evenings he was asked to curate to end the South Bank's Ether festival. Greenwood arranged “Arpeggi” for orchestra, and “Where Bluebirds Fly” too, the “There There” B-side the band used as intro music at their 2002 shows and had never played live. He played the ondes Martenot, an early electronic keyboard, behind Yorke, with the Sinfonietta and members of the Nazareth Orchestra, a visiting Arabic orchestra, on stage together for both songs. The singer Lubna Salame joined Yorke on “Where Bluebirds Fly”, singing without words. Monday's repeat, with Colin Greenwood in the audience, brought no surprises.

Jason Arber of Pixelsurgeon gave the Sunday 8 out of 10. Yorke walked on to loud applause looking ill at ease, he writes, and his voice was thin at first, growing as the arpeggios built; he sang with his eyes closed, playing an imaginary keyboard.

Within a day or two fan recordings of both songs, a short video of “Arpeggi” and a recording of “Smear” were being passed round. Reviewing one for Pitchfork, David Raposa places the ondes Martenot somewhere between a Rhodes piano and a pipe organ. The orchestra comes in slowly, xylophones and trumpets first, then bowed and plucked strings, and the song ends with Yorke repeating the word “escape” over a plucked two-note figure.

Greenwood chose the whole programme. His new “Piano for Children” was played on a piano whose upper register was retuned to quarter-tones, and “Smear” came back revised for strings, harp, horn, clarinet and two ondes Martenots. In the interval the hall watched a selection from The Most Gigantic Lying Mouth of All Time, the band's Radiohead Television films.

The hall was full of Radiohead fans, Stephen Pritchard writes in The Observer, and the whistling, stamping ovation at the end left Yorke and Greenwood looking abashed.

The programme, Royal Festival Hall, 27 and 28 March 2005