The idea of one of Pierre Boulez's works being spontaneously encored would once, perhaps, have been unthinkable. At the end of his third concert with the London Symphony Orchestra, the audience's enthusiasm for his performance of Notations was such that he returned to the platform and repeated the second of them.

Notations first saw the light of day as a series of 12 piano pieces, written in 1945. Boulez reworked five of them for orchestra in the late 1970s, when he was conducting Wagner's Ring at Bayreuth and didn't, as he told the audience, have much time for composition.

For the performance, the piano originals were placed alongside the later versions. "I hope," Boulez joked, "that you'll hear some similarities."

In fact, the differences proved almost shocking. The piano pieces, scrupulously played by John Alley, are brief, aphoristic clusters of sound. Each of the orchestral Notations, however, takes the original as the starting point for a joyous, thrilling exploration of rhythm and sonority. The orchestral colours recall Bartok, Berg and Messiaen, while the multi-layered polyrhythmic clashes peer back to, and elaborate Stravinskyan methodology. The playing can only be described as dazzling.

Bartok's Four Pieces for Orchestra and Rihm's Gesungene Zeit formed the first half of the concert. The Four Pieces, composed in 1912, are pivotal works in which late-Romantic thematic expansion combines with modernist compression and rhythmic savagery. Rihm, meanwhile, is the great subjectivist of German contemporary music, and Gesungene Zeit is effectively a violin concerto written in 1992 for Anne-Sophie Mutter. It consists of a single vast span of unrepeating violin melody that seems to breathe life into the orchestra, which then threatens to overwhelm it. Mutter has played it with more intensity on previous occasions, but in Boulez's hands the pulses and throbs of Rihm's orchestral writing have rarely sounded so beautiful or so dangerous.