When I was at school I remember hearing a live broadcast of Carmina Burana from the Proms; and though I barely knew the music, I knew enough to realise that the sudden disappearance of the baritone soloist, followed by a loud crash, wasn't in the score. The soloist had fainted; the performance was stopped (with an appeal to any off-duty baritones in the audience to step forward); and it marked my introduction - inauspicious but memorable - to the art of Thomas Allen, who thereafter became a fixture in my listening life. The compleat baritone with altitude (sic), he had the versatility, vocal compass and dramatic gifts to command a wide range of work; therefore many of my first encounters with vocal repertory were through him. In opera, oratorio and song, his rich-toned musical intelligence set benchmarks, as it seemed to me. No doubt it was the same for anyone discovering opera in the Seventies who was lucky enough to catch the run of Mozart baritones - Papageno, Figaro, Don Giovanni - that he sang at Glyndebourne, or the Billy Budds he sang everywhere else, not least at Covent Garden.

This week he celebrated 25 years at the Garden with a Don Giovanni which is the first in a series of stagings specially scheduled to mark the anniversary. Unfortunately it's a revival of the 1992 Johannes Schaaf production, which has never worked well, It's unlikely to be seen again, if Jeremy Isaacs's recent comments on the whole Schaaf Mozart-cycle are anything to go by ("I had it foisted on me" was their essence). With pitch-black sets, poor lighting and clumsy scene-changes, it narrates the story badly. And this revival suffers from a sensitive but dull conductor - Dietfried Bernet, substituting for Sir Charles Mackerras - with a British-based cast that looks good on paper but which doesn't deliver. Felicity Lott's Elvira and Anthony Rolfe Johnson's Ottavio never quite come into focus. Yvonne Kenny's Anna has a beautiful liquidity but not enough weight. And only Alison Hagley (Zerlina) and Robert Lloyd really go for their roles.

But that just clears the decks for Thomas Allen, whose night it becomes without question. Don Giovanni is his calling card. And where long-term relationships between singers and roles sometimes atrophy, handcuffed to a fixed conception of what was once a good idea, Allen's Giovanni grows in stature and refinement, with a menace that extends beyond sexual libertinage into voyeuristic torment. This particular Giovanni barely leaves the stage: he lingers on in shadowy corners, savouring the consequences of his actions like the animateur to a private theatre of distress. He pulls up a chair to get a more comfortable view of Anna's tears over the body of her father. And whether the initiative for all this comes from director (in which case it's one of his more inspired contributions) or singer, Allen becomes the prowling spectator with absolute conviction: a connoisseur of other people's pain who in another time and place would be commanding transports to Auschwitz or administering justice in dubious South American regimes.

He was also singing well on Monday night, with a firmer, fuller tone than last time round (when he seemed in ominous vocal trouble), and a relish for language that delivers Da Ponte's Italian with much the same synthesis of energy, clarity and finesse that you'd hear him afford AE Houseman's English in a recital. In all, this was a Giovanni to file alongside Allen's suave, Armani-suited Don Alfonso in the last Royal Opera Cosi, or his chasteningly poignant Beckmesser in the Garden's Meistersinger: different animals, but equally alive. The Beckmesser returns this season as part of Allen's anniversary series, and alongside it will be new, potentially uncomfortable character roles in Massenet's Cherubin and Pfitzner's epic Palestrina. Nobody could say this singer sits on his success.

Throughout the 1980s Thomas Allen was one of the handful of serious international voices who maintained a regular relationship with the Coliseum; and, without slight to the week-in, week-out excellence of ENO, it has to be said that the occasional presence of someone in that league does make a difference - as it does in the current revival of Rigoletto, which has Janice Watson as the Gilda. It's her debut in the role, and she's a joy to hear, with an endearing but not winsome innocence, a bright, clear top, and a secure technique that underpins the whole performance. Peter Sidhom's Rigoletto was eclipsed (he had apparently been ill, and wasn't in good voice or presence: there was very little fire in the attack), and so was just about everything else - except Jonathan Miller's New York Mafioso staging which, after 14 years of come-backs, is as on-target as ever. The West Side Story-style sets still cast their melancholic, Edward Hopper spell. When the overture ends and the curtain rises on Miller's gangland bar, the below-the-belt surprise of cultural disorientation never fails. And the glimpsed world of off-centre detail - bent policemen turning blind eyes, gangsters' molls knowing their place - is endlessly fascinating. ENO periodically threatens to retire this Rigoletto, but I can see it doing business until the sets disintegrate.

A few subway stops from the real 1950s Little Italy, you'd have found two composers independently writing small-scale stageworks about mid-life insecurity and what happens when the American Dream wakes up. One, a jewel- like 10-minute comedy, was Samuel Barber's A Hand of Bridge; the other was Leonard Bernstein's Trouble in Tahiti, a tragicomic 45-minute vaudeville which Bernstein later enlarged into the full opera A Quiet Place; they both played this week at the Southwark Playhouse, in highly effective little productions by Paul Baillie that got a lot out of very little in the way of physical resources. They underestimated the true operatic claims of both the pieces by casting them with too many West End voices: the intonation of the sardonic, smiling scat singers in Tahiti needs to be razor-sharp for the joke to work. But Dinah, the troubled, disconnected housewife Bernstein almost certainly modelled on his own mother, was impressively done by Lori Isley Lynn: an American whose voice and career-credits resist categorisation but amount to something striking that I'd be pleased to hear in grander circumstances.

Finally a note on the Clerkenwell Music Series, an off-the-beaten-track festival run by composer Roger Steptoe (who used to organise the superb composer-profile weeks at the Royal Academy of Music) that's in its stride now after two years of tentative growth. The 1996 themes are England and Denmark; the programmes are brilliantly devised; and Thursday's, which included Nielsen's faux naif, adorable Springtime in Funen, a rare shot at Delius's Seven Danish Songs, and a UK premiere by the contemporary Dane Bo Holten, was one of the most fascinating concerts I've heard in months. There's more to be said about it (and about the series in general), which must wait until next week, when things culminate in a celebration for the 85th birthday of Ursula Vaughan Williams. But I want to flag it here that Clerkenwell is turning into something special; it's worth investigating.

`Don Giovanni': ROH, WC2 ( 0171 304 4000), continues Tues & Fri. `Rigoletto': ENO, WC2 (0171 632 8300), Sat. Clerkenwell Music Series: Holy Redeemer Church, EC1 (0171 228 8546), to Sun 17 Nov.