WITH its "Omniscient Seashell" and gratuitous invasion by characters who seem to have escaped from a Rudolph Valentino movie, Strauss's Die Aegyptische Helena ("Egyptian Helen") has the silliest of modern opera plots. It also has a peculiarly Teutonic sense of irony: one which doesn't travel. Nor did it to Britain, until last summer, when Garsington boldly undertook the UK premiere, so writing itself into the annals of opera history with a very creditable performance.

But to get the full measure of what is actually a sumptuous, superlative- defying score, you had to wait until last weekend when the Royal Opera gave it in concert at the Festival Hall. This London premiere was an exotic musical experience, with a mixed-bag cast, but two outstanding stars: the soprano Deborah Voigt and the conductor Christian Thielemann.

Voigt, singing the title role, was stunning: a capacious Wagner voice of liquid gold, as radiantly lyrical as it was strong and focused. As for Thielemann, his control, authority and confidence were absolute. This is a young conductor, tipped by many (not least Deutsche Grammophon, his record company) to be tomorrow's Karajan; and we've all heard that one before. But Thielemann genuinely has Karajanesque potential, certainly in German repertoire, and what he did here was electrifying, with the RO orchestra on top form, disciplined and powerful like a mighty engine. What a show.

The pity is that this Helena brings to an end the series of concert- shows which have been the salvation of the Royal Opera during its past annus horribilis. According to the new schedules - which the company has just quietly slipped into circulation - it plans nothing more in London (apart from a single Ring revival, semi-staged at the Albert Hall) till mid-December. Yes, that's mid-December! No doubt the RO will fend off accusations of lethargy by pointing to a touring schedule that includes Edinburgh, Baden-Baden, Savonlinna: anywhere it can grab some money. But that's the desperate depth to which the Kirov Opera is reduced: forever on the road, and never servicing its home audience. To offer London nothing new until December is pathetic.

Maybe Glyndebourne should throw together a quick London season to fill the gap, leaving behind its new, profoundly disappointing Cosi but bringing (please) its handsome revival of Kat'a Kabanova, which opened on Saturday. I always find Kat'a the most completely satisfying of the Janacek operas: tight, incisive, but enlarging into heartfelt lyricism. Nikolaus Lehnhoff's production is a classic response to those qualities, small in scale but big in impact. And though it's 10 years old, it's just as sharp and vivid as before, with Tobias Hoheisel's deep-dyed sets as lurid and entrancing.

The surprise this time is Amanda Roocroft in the title role. She's no dream-cast Kat'a: the voice is too hard. But it's also rather beautiful, with an intense dramatic power. The veteran Helga Dernesch, a Bayreuth star of the 1960s and 1970s, is an imperious (but in the drunk scene, rather racy) Kabanicha. And Kudrjas is idyllically sung by Timothy Robinson, a young British tenor who has been nursed by the RO and is now really coming into his own: he has a lyric voice of charm and possibilities.

Organists in Britain are a twilight species, spotted (back view) from afar in organ lofts and rarely making news until they get the sack. In France, though, there has always been a grand tradition of the maitres d'orgue. And through the 60 years that Messiaen was Titulaire at the Trinite in Paris, crowds assembled in their thousands to hear him, not least on the fabled occasion when he gave the French premiere of his Livre d'orgue and had to fight (literally tearing his clothes in the process) to get into his own recital.

Some of that excitement seems to have transferred to Westminster Cathedral in the past few weeks where, every Tuesday night, Dame Gillian Weir has been working her way though Messiaen's complete output for the organ. I haven't noticed any torn clothes, but the crowds have been amazing, turning these recitals into the most momentous event the British organ world has seen in years. And from the rapturous standing ovations they've drawn, they clearly count among the most momentous performances too.

The organ has a central place in Messaien's sound-world. He was writing for the instrument throughout his life, although in the post-war years, as his relationship with Yvonne Loriod developed, so did a greater interest in the piano. From the early L'Ascension, Nativite du Seigneur and Les Corps Glorieux of the 1930s to the late Livre du St Sacrement of 1984, the scores came sparingly but massively, in great contemplative blocks that tower above anything else written for organ in modern times.

The Livre du St Sacrement lasts nearly two hours. The Meditations sur le Mystere de la Ste-Trinite, which Dame Gillian played this week, takes a good hour and a half. And the problem with each one is to experience and feel it as a total integrated work. Which isn't easy. Messiaen's music moves without developmental linkage. It parades "found" objects - plainchant, birdsong, a tone-cluster, a number-game, more birdsong - which are laid out like a group of precious items on a nursery-school study-table, and with much the same degree of innocence, however subtle the theological and symbolic agendas the composer sets himself.

To make this music work requires, in a sense, an act of faith. But more practically, it demands an understanding of the devotional context, a feel for timing (Messiaen fervently believed in the expressive potential of durations), an ability to reconcile the grandeur with the modesty of Messiaen's vision, and an ability to energise the heavy structures that support it. So far, these performances have had the lot: magnificently, with illuminating liturgical contributions from the Cathedral Choir under James O'Donnell, and a powerful vitality in the playing. Weir's idea of Messaien isn't (as she'd say herself) a warm bath of religiosity. She plays with brilliance, muscle, and a resounding sense of joy; and with an ear for registration that exploits the warm reeds of Westminster Cathedral's nave organ (a Harrison, but close in capabilities to the sort of Cavaille-

Coll monster that Messiaen loved) without smothering the detail. It's a privilege to hear, a tonic to the soul.

Some soul-medicine by John Harle premiered at the Salisbury Festival last Sunday: at least, Salisbury says it was a premiere. But the BBC calls it a "preview", on the grounds that the score - a chamber opera called Angel Magick - has been commissioned by the Beeb for this year's Proms.

Premiere or preview, it's about the Elizabethan alchemist John Dee, with a David Pountney libretto as densely allusive as Harle's score, which is an eclectic fusion of ancient (including a consort of viols) and modern (guitars, drum-kit) with bits of jazz, Tudorbethary and easy-listening modernism circumnavigating each other in a diverting dance of styles.

All that's missing is a virtuoso solo sax (John Harle, alas, conducts the piece and doesn't play in it). But otherwise it treads the sort of crossover ground in which Harle struck gold before with his Terror and Magnificence disc. My one reservation was whether it's really for the stage. I'd call it radio drama, or maybe an erudite form of radio cabaret; which means that however it looks at the Albert Hall (probably lost in space), it should at least sound well on air.

'Kat'a Kabanova': Glyndebourne (01273 813813), Wed, Fri, Sun. Messiaen (0171 798 9057): every Tues to 16 Jun. 'Angel Magick': Proms (0171 589 8212), 21 Jul.