MUSIC / Top marks: Stephen Johnson reviews the London Sinfonietta at the QEH

THERE has to be a question mark over the format. Two 20- minute intervals in a four-work programme - with only one of the works featured significantly longer than 20 minutes - seemed pointlessly extravagant.

But in every other aspect the opening concert in the London Sinfonietta's 25th Anniversary season was a roundly affirmative experience. Here was a fascinating, cleverly assorted sequence of tough contemporary or near-contemporary British works, culminating in one of the toughest of them all, Harrison Birtwistle's Verses for Ensembles - in its structural preoccupations recognisably a product of the late 1960s, but here truculent, gritty and uplifiting as ever. The contrast with the preceding item (still sharply memorable after interval number two) was pronounced, but in some ways Nigel Osborne's Choralis prepared the ground for the Birtwistle rather neatly. If Verses for Ensembles lays bare the elemental forces of ritual, Choralis shows the emotions that cluster about it: joy, awe, love and lament, presented in sharply distinct folk and liturgical colourings drawn from cultures near and very far. But there's more to Osborne's sinewy re-working of his source material than cultural pot-pourri, as the London Sinfonietta Voices' stylish performance revealed.

Chronologically closer to the Osborne than to the Birtwistle, Peter Maxwell Davies's A Mirror of Whitening Light seems remote from both but just as single- minded. Textures come close to opacity in places, but there's a strong expressive charge, and a driving force behind it all - the kind of impulsive thinking that has often seemed disappointingly sporadic in his more recent pieces. The use of plainsong is audibly significant (even to those who don't meet Davies's ideal of 'the sharp listener who knows his Liber Usualis') and the take-off from and eventual return to the pitch C makes a clear, vital musical argument. Again, strong, characterful playing by the Sinfonietta was an important factor in final success.

But perhaps the biggest achievement of the evening was the finding of a new work that could carry itself confidently in company like this. By calling his Sinfonietta piece ], Benedict Mason intended, or so he says, not an exclamation mark but 'a phonetic symbol for the alveopalatal click produced by pulling the tongue sharply away from the front of the hard palate'. Maybe, but for me the single exclamation mark will do. At times ] sounds as though Satie, Ives and the crazier Percy Grainger had got together for an evening of chemically-heightened table-talk. Strands of half-familiar ideas flash past, sparking erratically, dancing or intoning enticingly, but avoiding any sense of forward movement - purposeless and proud of it.

The end of ] - final only in the sense that nothing follows it - is spectacular. One by one, most of the players rise from their places, swinging Schwirrbogen - instruments that look like windscreen- wipers on the end of divining rods, and make a sound rather like their name. They slowly walk through, and out of the auditorium, while offstage the percussionist beats slow, regular strokes on a gong. As with Haydn's Farewell Symphony, one instrument is left - a heckelphone playing a slow, dirge-like line to conductor Elgar Howarth. Does Mason's riddle have an answer? To find out, we will have to hear it again - soon, please.