A heartfelt plea for Clemenza; MUSIC

We All Know La Clemenza di Tito as the opera in which Mozart, in the last year of his life, revisited the old, dying-on-their-feet conventions of baroque opera seria he abandoned years before. And so we think of it as musically regressive, staid, a touch dull: inexplicable as the exact contemporary of Zauberflote and the Requiem. That Mozart has improved on the conventions, slimming down the arias and sharpening the pace, eludes us. And while Titus's grapeshot forgiveness for everyone in the cast who has abused his kindness, torched his capital and tried to murder him may have struck 18th-century audiences as clemency, to us it seems more like a failure of imagination. Not much substitute for the diverting spectacleof vengeance.

Titus is certainly a difficult central role to make anything of. Written for the singer who premiered Don Ottavio in Giovanni, it has the same relentless virtue which, however fine in real life, is a bore on stage. But in the very staidness of Clemenza there's a kind of muscle which occasionally some magic combination of director, singers and conductor finds and flexes. And that's exactly what happens in the new Welsh National Opera production: a handsome and distinguished staging of the kind WNO periodically manages to pull out of its hat.

Yannis Kokkos, the director/designer, follows his own precedent fromWNO's famously successful Tristan with a simple (though actually sophisticated), clean-cut, sculpted set, and strong but understated movement. You might ask for more at the end of Act I, in which Rome burns rather coolly and the reported assassination generates only the most decorous turmoil. But decorum is the whole point of this show. Its tensions are driven by an engine of restraint. And driving the musical input, Sir Charles Mackerras is magnificent, proving himself yet again as a mature Mozartian who doesn't sit on his wisdom, but uses it as licence to explore and dare. His judgement is impeccable. His sense of style is exact. And I particularly like the way he profiles the importance of the clarinet accompaniments, bringing the player up from the pit and into a stage box for the big set-piece obbligatti.

I also like his choice of singers. Glenn Winslade doesn't claim the stage as Titus, the voice a bit woolly, but it's attractively done. Isabelle Vernet's Vitellia lacks spit and fire, but has a commanding presence. And there are wonderful performances from Katarina Karneus (a lithely vibrant Sextus), Lisa Milne (a melting Servilia) and above all, Paula Hoffmann - an entrancing Annius, with a big range and an all-round vocal promise that should earn this Swedish artist frequent visits back to Britain.

William Christie's Paris-based Les Arts Florissants visited Britain last weekend for a four-day residency at the Barbican. The theme was baroque music theatre; the composers Monteverdi, Purcell and Charpentier (from one of whose operas the Christie gang take their name); and the performances I heard were a delight, if slightly precious.

I can only take so many of the nods, winks and self-satisfied smiles that effectively were the semi-staging of Charpentier's Les Plaisirs de Versailles: a short, allegorical salon-piece in which Music, Conversation and a bunch of other living abstracts squabble over precedent and toady to the king. Exquisite, dainty entertainment of the kind that would have prevailed in the royal salons of mid-17th-century France, it offered an interesting historical precedent for the period pastiche of Strauss's Cappriccio (complete with jokes about drinking chocolate), and it was nicely sung by Christie's small company of light, young voices. But its thin inconsequentiality suggested why Charpentier has always struggled for position in the hierarchy of the baroque: perpetually overshadowed by Lully, his great contemporary.

There was far more meat on the bones of Purcell's King Arthur, necessarily the highlight of the Christie weekend. Two years ago Covent Garden played host to Les Arts Florissants for a fully-staged King Arthur as part of the Purcell Tercentenary, and it raised the enduring question of how you stage these 17th-century semi-operas with their reams of less-than-stunning spoken dialogue. At the Garden we got everything: an embarras de richesses. Here at the Barbican the dialogue was reduced to brisk narrations by a pair of actors. It was a test of attention to follow their multiple roles as observers of/participants in the story, but it was a viable solution, virtuosically handled by Philip Franks and Rebecca Saire, with small-but- perfectly-formed vocal performances from a cast which included Sophie Daneman and Paul Agnew.

As if there wasn't enough baroque theatre at the Barbican this week, the Royal Opera weighed in with the London opening of the Rameau Platee which they premiered at the Edinburgh Festival a few weeks ago. It has improved. Considerably. The jokes are tighter, sharper, funnier. And though it still seems to me that Mark Morris's energies have been channelled into directing the dancers (where he's on sure ground) rather than the singers (where he isn't), things have largely settled into place. Susan Gritton, Francois Le Roux, Diana Montague and Mark Padmore all make memorable contributions to what is now a truly entertaining company show. At its heart remains the brilliant conception of Platee herself, the unmarriageable bog-nymph with attitude. Jean-Paul Fouchecourt's high-tenor travesty performance in Isaac Mizrahi's crazy frog costume is a perfect synthesis of the grotesque and almost-loveable. The character should sell to television. With the merchandising spin-offs it could keep a homeless opera company off the streets.

Meanwhile, and still at the Barbican where all roads lead these days, Tuesday saw the start of the London Symphony Orchestra's new season with a concert that felt like a statement on where and for what the LSO currently stands. The first half was Beethoven's Violin Concerto (= re-establishment of territorial claims to core repertory) taken at patrician speeds but with extreme refinement under Colin Davies, and with Midori as a committed if introvert soloist. The rest was Walton's 1st Symphony (a lot more Walton coming up this season), in which Davis drove the nagging rhythmic underlay into a tough, hard-bitten English sound a world away from the well-oiled American machinery the LSO became for Michael Tilson Thomas. I'm not sure which I'd choose for preference. But the Walton should be good.

`Clemenza': Cardiff WNO (01222 878889), Fri. `Platee': Barbican, EC2, (0171 304 4000), Tues & Fri.