Knussen at 50/London Sinfonietta, Queen Elizabeth Hall, London<br></br>Sweeney Todd/Opera North, Sadler's Wells, London

Where were The Hecklers on Wednesday night? No, I'm not referring to some obscure Indie band. I mean the anti-modernist, pro-melody protest group who took out an advertisement in The Spectator to recruit boo-ers for the 1991 première of Birtwistle's Gawain. Have they disbanded? Or did the Queen Elizabeth Hall's concrete carcase act as crucifix and garlic clove? Because give or take a couple of their favourite hate-figures, a veritable tone-cluster of contemporary composers were gathered there for the 50th birthday of Oliver Knussen. And had The Hecklers sat through the London Sinfonietta's long celebratory programme of juvenilia, greatest hits and world premières, they would have witnessed a post-performance identity parade to haunt their dreams; there, lined up like a compositorial coconut shy, stood Knussen, George Benjamin, Alexander Goehr, Mark-Anthony Turnage, Colin Matthews and Magnus Lindberg. Overwhelming? You bet. But the only things to be thrown were cheers.

Only a fully paid-up Heckler could really complain about Wednesday's programme – much of it fascinating, some of it ravishing, quite a lot of it as pro-melodic as you could get, and all expertly played – but it was rather like crashing a private party. Ten changes of ensemble set-up made for many games of Musical Chairs, and, as a result of an unadvertised change in order, Pin the Piece on the Composer was compulsory. But Elliott Carter supplied the first smile with his agitated bassoon/viola dialogue Au Quai (drawn from a joke by that well-known comedian Arnold Schoenberg), while Turnage's sweetly sassy Snapshots lifted what became a rather heated home-stretch to Lindberg's vigorous Bubo bubo. And if a total of eight telegrammatic two- to four-minute world premières from these, and other, composers left this listener punch-drunk, it also left her impressed. The focus was, of course, on Knussen; with works from Masks (1969) – a Stockhausen-esque walkabout for solo flute and chimes – to 1994's electric bright/soil-spattered Two Organa, by way of the lucid, lyrical Songs without Voices and Elegiac Arabesques (1991-2), and Océan de Terre (1972), the vocal line of which – attractively sung by Claire Booth – hints at Jennie the Highland Terrier's tender, frustrated themes in the second of Knussen's Sendak operas Higglety Piggelty Pop! If nothing else, this lap of honour retrospective made me realise why Knussen's music has been undervalued: that much of what he writes is so easy on the ear and cherishing of the heart has occluded how rewarding and demanding it is of the brain. Happy Birthday, Mr Knussen, and bravo.

In his Film Studies column last year, David Thomson reported more chills from San Francisco Symphony's concert performances of Sweeney Todd than Scary Movie 1 and 2. If only Stephen Sondheim had found Alfred Hitchcock, he wrote, comparing Sweeney Todd's dramatic clarity to that of Psycho. (In fact Sondheim pays homage to Bernard Herrmann, composer of the score to Psycho, through a succession of "Herrmann chords" in Sweeney Todd.) But that was San Francisco not Islington, and not Opera North's touring revival of David McVicar's 1998 production, where, instead of Hitchcock, we have an uneasy tribute to David Lynch. Not the emetic phantasmagoria of Eraserhead, but the calculated, mechanical carnival of The Elephant Man.

I suppose one should be grateful that the influence here was gothick advertorial rather than sci-fi – Sweeney Todd in the style of Dune, anyone? – but, with the exception of the gore-stained drop-cloth that closes Act I, McVicar's production has aged as badly as Lynch's film. Though Opera North's chorus and orchestra do an efficient job with Sondheim's score – spitting the staccato cries of "Fleet! Street!" with stridently sibilant received pronunciation – their performance underlines the problems with treating musical theatre as though it were opera. As interesting as Sondheim's characterisations are, as witty as his libretto is, and as Bergian as his harmonies can be on occasion, the show-stopping tunes are exactly that: caesurae in the narrative of the score, and liftable enough to fill Marie's Crisis on a rainy night. OK, you could say the same of Tosca. But add to this the fact that the anti-legato delivery required in A Little Priest or By the Sea involves the conscious exploitation of vocal gear-changes – the very register breaks that conservatoire tuition attempts to eradicate – and it's hardly surprising that among Opera North's classically-trained principals, the only performer to energise, engage and excite was actress Beverley Klein (Mrs Lovett). Sweeney Todd ain't opera any more than Bohemian Rhapsody is a madrigal, and neither Stephen Sondheim nor Opera North are served well by this match of company to work.

a.picard@independent.co.uk

'Sweeney Todd', Opera North, The Lowry, Salford Quays (0161 876 2000), 18 & 22 June, and touring to Nottingham Theatre Royal (0115 989 5555), 25, 26, 29 June, Theatre Royal, Newcastle (0870 905 5060), 2 & 3 July, Lyceum Theatre, Sheffield (0114 249 6000), 10, 11, 12 July