OPERA / Two cheers for Harry: Never mind the stunts - how did Gawain go on the night? Robert Maycock was at Covent Garden

Forget opera; think ritual and myth. You have to be there to get the hang of Sir Harrison Birtwistle's Gawain, and it doesn't have much to do with the kind of show that sings its way into a million hearts. Music is at the centre, of course, but it takes its place as one element amid the visual spectacle, the dramatic confrontations, the poetry and the ceremony.

The story is the starting-point, the legendary encounter between Gawain and the Green Knight. The Round Table courtier takes up the Knight's challenge to behead him, only to find the severed head insisting on a return match, a year and a day later. Gawain keeps the appointment but is caught cheating. While the Knight seems able to forgive an attempt to save one's skin, Gawain has to live with the knowledge of his cowardice.

There are hard confrontations and harsh words. Beyond the telling of the story there are cycles of the seasons, games and trials, recurrences and revisits. Events mysteriously happen three times over. It's easy to see why this fusion of Northern bluntness and rite-like enactment should catch Birtwistle's imagination. For the Royal Opera it is often beautifully visualised by Alison Chitty's designs and brought to spacious life in Di Trevis's production.

The definitive moment is the arrival of the knight on his horse, a coming together of musical and theatrical strands that have been a long time in the tightening. What follows is horror movie and knockabout rolled into one. Like the ultimate pantomime horse, the beast wobbles decorously downstage as the brass neigh at full volume and the sound of a 17-legged gallop breaks out on wood blocks. From the saddle the voice of John Tomlinson roars forth and sends the temperature of the whole performance into instant ascent.

It isn't pretty to the ear. In the theatre the sheer navety of the musical gesture is audacious enough to do the trick. A grim humour crops up again in the thumps and screams with which the orchestra underpins the three hunting scenes. Some of the score uses a pictorial literalism as simple as the cuckoo-calls in Beethoven's Pastoral Symphony, but once you are on its wavelength it offers plenty of unexpected fun.

Other things work their way under the skin more insidiously. The growl, thud and creak of Birtwistle's tubas and bass drums, the scrambling rise and screeching arrival of clarinets in flocks, the muffled jangling of a cimbalom, give the sound a distinctive colour. At the opposite extreme from the nave, Birtwistle's interweaving of hunting and seduction scenes, with rampant horns a suggestive link - shades of Berlioz's 'Royal Hunt and Storm' - is done proud by the staging's hectic chase across the back of the stage.

And, yes, there is rather a lot that doesn't add up. The orchestra swamps the singers, the singers cover each other, the musical lines don't always help the words in the first place: try singing 'is tested by' in half a second, over and over, at any pitch. Vocal lines are monotonously drawn out. There isn't much variety of pace either, and it takes a long time for dramatic momentum to build.

Now that the end of the first act has been rewritten, it gives the action a powerful forward push. But it has the effect of displacing the longueurs, so that the opening and the repeated encounters of the second act now feel out of proportion. I like the way David Harsent's libretto poses riddles and brings them back when you know more - 'This is the moment that waited for you as you journeyed towards it' starts to make sense the third time around - but they don't always work. No matter how often the offer crops up, I don't know how to choose between legacy and loss, let alone vanity and choice.

On Thursday's first night it got half a standing ovation from a well- filled house, following a stifled Sloanish cry of 'Shame about the score'. A few thin jeers percolated through. From behind me, a quiet booing kept going with heroic persistence even though it reached only a couple of rows. Well, at least somebody heard; the source turned out to be Keith Burstein, the Romantic Futurist, who last week was refusing to go along with the tactics of what he called 'an extreme right-wing protest group'. Birtwistle gave a victory salute from the stage. Why did he bother? Was he rattled?

Strangest sight of all was two young composers of different conservative shades, the 'Heckler' Frederick Stocken and the modernist George Benjamin, picking each other apart on Newsnight like a tiff out of La Cage aux Folles. They ignored the Goreckis and the Taveners and the joyous pluralist mix of cultures and styles that make up the full texture of contemporary music and give the lie to the whole hyped-up affair. Who will protest when ENO premieres Judith Weir's Blond Eckbert next week? For most younger musicians Birtwistle isn't even contemporary, and Gawain a magnificent dinosaur, although this may be news in some still powerful quarters. If the protesters really want to hit home they should go and picket university music departments; but that won't get into the papers.

There are other issues lurking unresolved: why, in stingy times, should a cash-devouring monster survive when the Garden Venture can't? Ironically, though, Gawain is the wrong cause to fight over for another reason. Neither the best nor the worst of Birtwistle, you can't defend it as an unalloyed masterpiece without being unfair to Earth Dances or Ritual Fragment. Yet the staging, in the round, is still a consuming experience. It's full of other fine performances: Francois Le Roux's Gawain, who somehow can get the words across nearly as well as Tomlinson, the soaring, full-stretch Morgan le Fay of Marie Angel, Anne Howells as a persuasive Lady de Hautdesert, Omar Ebrahim's manic Fool. Catch it now; you will not see its like again.

Further performances 7pm today, 20, 22, 28 April (booking: 071-240 1066). Radio 3 relay 28 April

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