LSO/Boulez, Barbican, London

Some composers seem to spend their lives writing the same work over and over - think of Mahler's symphonies. But why start from scratch when you can improve an old work? The attitude has usually been that of an improviser rather than that of a classical composer, and then came Pierre Boulez and his "works in progress". Unlike an improvisation, the notes are written down, yet the piece is never definitive and may continue to be developed for a lifetime.

So, when Boulez brought Dérive 2 to an ensemble of players from the London Symphony Orchestra, it was at the same time one of his most recent works and one whose roots go back more than a quarter of a century.

Dérive 2 is unlike the sensuous, hyper-reactive Boulez of 30 or 40 years ago. It amounts to nearly half an hour of continuous pace, and has the momentum and rhythmic energy that steadily infiltrated his music in the later decades of the 20th century, yet never so fully. It's as dense and intricately worked as ever, but the overall sense of direction imposes itself more immediately on the ear. You even see Boulez beating four in a bar. Against it, the syncopations of marimba and piano sound as though they aspire to a state of spontaneity, however minutely detailed they are on paper.

The frantic action starts from a sustained note, and the harmonic basis emerges by stages until the horn finishes as it had begun. It's a ferocious test of players' concentration, the more so for staying quiet much of the time, but Boulez drew some sparky, propulsive playing from the members of the LSO - never mind that some of them looked as if they had aged several years by the end.

When the full orchestra assembled for Mahler's Symphony No 7, it took some while to find its usual precision and balance. This is the symphony that develops features of the previous two, along with giving its composer's latest thoughts on progress from dark to light, yet constantly goes off in quirky directions that Mahler never pursued again. The strange harmonic pile-ups from intervals of the fourth give it a unique, astringent quality. The performance was severe, monumental, and innocent of charm - innocent in that it shunned the knowing, ingratiating turns of phrase that often pass for being "Viennese". Instead, there was a direct, ruthless focus on the individual lines within Mahler's sound tapestry, and, on the way, they add up to a powerfully cumulative whole.

The experience was far removed from the opera of the soul that most conductors make of a Mahler symphony, and this transfer of vision from inwards to outwards paid off best in the massive symphonic statements that begin and end the work. Boulez had written in the programme that he found it difficult to make the finale all of a piece, but in the pacing, he had surely discovered a way.