George Benjamin first appeared as a cinema pianist during his student days, when he improvised a score for Lon Chaney's silent classic The Phantom of the Opera. As part of his own By George! series at the Barbican, he has returned to silent movies, providing music for Georg Pabst's 1928 masterpiece Pandora's Box. As a tour de force of concentration and stamina, his improvisation is remarkable. Whether it captures the film's essence, however, is open to dispute.

Pandora's Box is, for many, the apogee of Weimar Republic cinema, drawing on cabaret, expressionism and, above all, the sharpness of the New Realism movement of the late 1920s. Benjamin's primary musical influences are French, however, and his improvisation is saturated in impressionist fluidity, much of it sounding like Debussy.

Benjamin is also no eroticist and too much of his score is on the wrong side of the divide between the sensuous and the genuinely sensual. The best parts come in the last half-hour, when Lulu is on the skids. The swaying Habanera with which Benjamin accompanies the sequence on the illegal casino-ship perfectly encapsulates both the vessel's rocking motion and the even more rocky emotions of the people on board. The horror and compassion of the final scenes are still unrivalled and Benjamin largely leaves Pabst to his own devices, punctuating them with sparse percussive throbs, before letting the final minutes tick away in silence.

The centrepiece of the main concert was Benjamin's Sometime Voices, based on Shakespeare's The Tempest and depicting the sounds of Prospero's magic island. Colin Davis conducted the London Symphony Orchestra and Chorus with beauty and savagery. William Dazeley enunciated Caliban's stuttering lines with rapture and terror. Benjamin flanks it with Stravinsky's Symphony of Psalms and three orchestral sections from Berlioz's Romeo and Juliet.

Both performances were exceptional, though the Berlioz extracts felt like bleeding chunks hacked from a whole.