THE SCENE: a dark, oppressive office building in totalitarian central Europe. Desks in rows, red flags and banners on the walls. An epic statue of Our Leader glimpsed through open doors, and men in uniform examining the contents of wastepaper bins.

Could it be Shostakovich, or a radical Macbeth? A Cold War Tosca? No ... it's ENO's new Elixir of Love, that Donizetti delight you always thought of as a jewel of pastoral charm and innocence. And you were wrong, according to the director Jude Kelly, who refuses to be seduced by the bel canto conventions of lovesick tenors, teasingly adorable soubrettes and roguish buffo basses, and wants to tell you that this opera is about political oppression, exploitation of the vulnerable, and the cruelty of social systems. So now you know.

What Kelly dumps on Donizetti, though, is a spectacular irrelevance. I've rarely seen an opera staging so in conflict with its own score. Not to say in conflict with itself in that, having buried the piece in a pall of gloom, Kelly then tries to resurrect it as a laugh-a-minute game-show. Desperate for gags, she gives her chorus D'Oyly Carte routines, and her Dulcamara an exploding circus car and a sort of New Age roadie wife. And the nonsense piles up, crass and contrived. It feels like ENO on bended knees wanting to be the People's Opera. But the People deserve better.

By way of consolation, the People did at least get a good musical performance, crisply conducted by Michael Lloyd with Barry Banks in fine, light-headed voice as Nemorino and Mary Plazas brightly cultivating those furtive tears as Adina. But I remember Plazas doing the same thing in English Touring Opera's old Elixir. And that simple, stylish, genuinely fresh-look show outclassed this ENO monstrosity on all counts.

A large part of the Barbican's music programming this year will be given over to a staggered series whose ambiguous title, "Inventing America", would worry me if I were the cultural attache at the US Embassy. America, it seems to say, is not quite real: it's a thing of mythic fabrication rather than substantial fact. And the season opened last weekend with a piece that celebrates the way Americans can't pick their noses these days without attributing iconic stature to the way they do it. (Not for nothing did they call him Bogey.)

The piece was John Adams's Nixon in China, an opera that plays like an extended CNN newsflash and depicts the meeting that took place between Chairman Mao and President Nixon in 1972 as a self-conscious exercise in making history. Nothing much occurs: it's just a diplomatic dance between two ageing men who happen to rule half the world. But mere banalities assume profound significance when half the world is watching you, evaluating every nod and gesture. And the brilliance of this piece lies in the way its music similarly enriches the banal, through minimalist processes: the repetition of small groups of notes until the listener is either mesmerised or driven mad.

I say "brilliance" with some caution, because minimalism can be crude and lazy, a cheap tool to drug the ear into submission. But John Adams is the genius of the genre, the one who has used it most virtuosically, most analytically. In Nixon, a score which is now 10 years old, he builds dynamic textural complexity from microscopic shifts in emphasis within the repetitions. It's a trick, but of Stravinskyan cleverness. And the result is music in a constant flow. It dances on hot coals, exhilarating and exhausting. While the orchestral machinery churns underneath them, the voices spin long lines from arpeggiated figures, stressed by Adams's tendency to press-gang lower voices (mostly baritones and mezzos) up to pitches that exceed their comfort range.

The voices were generally wonderful, extracting character from writing which isn't always characterful. The veneer of glamour and underlying vulnerability of the Nixons was beautifully developed by Dietrich Henschel and Wendy Hill. Judith Howarth's Madame Mao was sensational in the blowsy showbiz number that sounds like a love letter to Shirley Bassey in Act II. And though conducting Nixon is more a matter of arithmetic than interpretation, Kent Nagano both sustained the excitement and nurtured the romance that somehow lives in this music alongside the mechanical engineering. The pity is that it was only a concert performance. Nixon has been staged throughout the world, but never by a British company, and never at all in London. That's a poor thing. It's a landmark score. Now we've heard it, we should see it.

There was a time when the London Sinfonietta seemed to have an anniversary celebration every six months. Michael Vyner, its exuberant then-director, enjoyed a party. And he never missed an opportunity to raise a glass to what he had achieved. Which was a lot. It was a flagship band, revered throughout the world of modern music, with the cream of British instrumentalists among its players.

Since Vyner's death, there's been less to celebrate. But none the less, the Sinfonietta is still here, still playing (handsomely), and still commissioning new work. Which is why the great and good of British music turned out in force for its 30th birthday concert at the QEH last weekend. A Janus- like event that tried to look back and forwards at the same time, it started with five miniature commissions from young composers intended to run in sequence and, perhaps as a result, cautiously written - as though none of the authors wanted to stand out from the group.

Then came a reappraisal of the past: Toward the Whitening Dawn, written for the Sinfonietta in 1980 by Jonathan Lloyd, with a brand-new sequel, And Beyond, completed last year. Whether Whitening Dawn required a sequel is debatable: the Siamese-twin result ran longer than its new material justified. But the exuberance of Lloyd's eclectic style, touristically rampaging through the worlds of Britten operas, dance-band jazz, Elizabethan madrigals and evangelical religion, was a crazy pleasure. Bold, affirmative and utterly sincere. Which is more than I can say for John Tavener's The Whale, the landmark of coffee-bar avant-gardism written for the Sinfonietta's first ever concert in 1968 and repeated in this birthday beano. Rather dutifully. Tavener was only 23 when he produced it, his encounters with primordial religion still to come. And in his youthful vigour he throws everything to hand into the pot without particularly cooking it. Still less believing in it.

Michael Berkeley's new piece, Secret Garden, doesn't have that problem. Written for the LSO and premiered on Wednesday at the Barbican, it's all belief: a vision of some magic haven, whose encircling wall and fragrant plants are represented with exact correlatives in sound. And, as the garden is traditionally a symbolic place in which conflicts are resolved (remember Figaro Act IV), so Berkeley's garden resolves what has, I think, been a long-standing tension in his work between technique and feeling. Secret Garden is as elegantly written as it is demonstrative and graphic - beautifully performed under Colin Davis and appropriate to its purpose, which was to celebrate the 75th anniversary of Berkeley's publishers. OUP.

One more anniversary this week (then that's enough): the 10th birthday of the New London Orchestra, whose testing-but-tuneful programmes have carved a distinctive niche for themselves under their conductor Ron Corp. The event was marked on Thursday at Smith Square with two premieres: Glad Day, a rollercoaster concert overture by Richard Blackford, and a Brittenesque Piano Concerto written and performed by Andrew Simpson. Two new works in one night is an undertaking. That the NLO achieve such things in such times is a small but sterling miracle.

'Elixir': Coliseum, WC1 (0171 632 8300), Tues & Sat, and in rep to 17 Mar.