BBC SO/Robertson, Barbican, London <!-- none onestar twostar threestar fourstar fivestar -->

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Commissions for orchestras of unlimited size come so rarely that most composers welcome the temptation of a grand and, they hope, glorious noise. Simon Bainbridge has been there, done that two decades ago, in a Fantasia for Double Orchestra that still counts as a peak in his now substantial output. This time, the BBC's invitation has produced half an hour of refined, shifting orchestration and muted dynamics.

In his Diptych, Bainbridge has gone exploring the sheer diversity of subtle combinations available from a hundred musicians, and come up with music that fascinates by its quietly mutating colours and almost heroic restraint, as though confident that the story of what it all means can be told on another occasion.

Despite the name, Diptych is a work of two unequal parts, like a prelude and an elaboration. Bainbridge prefers that they be played separately, as they were at the BBC Symphony Orchestra's lucidly prepared premiere, one at the start of each half. Both take visual metaphors as their starting point: images that break up and reassemble in unexpected but coherent ways.

The first deals in quiet chords and even quieter articulations of the space between them, often just with faint percussion rolls. Horn colour strengthens the palette and achieves a temporary dominance, but there is no melody or pulse, just flurries of notes rippling the surface. Harmonies recall Debussy, particularly Jeux, but with an implicit sense of evolution that steals up as the music proceeds.

Part two sets out from similar stillness - separation makes it like a reminder rather than a continuity - but soon sustains a multi-layered concentration. It remains quiet, but activity is purposeful and eventually settles round a long melodic line that comes and goes.

Afterwards, even more horns came on for the Poem of Ecstasy by Scriabin, but the effect of Diptych encouraged you to listen to this music, too - a work that certainly doesn't resist the urge to go loud - as harmonic flux rather than a rather obvious metaphor for the build-up to orgasm. David Robertson, conducting, drew from the orchestra a fetching mix of refinement and fluidity.

This quality put in an effective showing at the end of Bartok's Piano Concerto No 3, when the orchestral brass and the soloist Barry Douglas egged each on through a pacy finale.