I remember once falling into a late-night argument with a friend about whether there was any real meaning in Mozart's Don Giovanni. My friend insisted there was not; I insisted there was. Typically, Mozart's clever ambiguities gave each of us enough fuel for our position, without ever dealing either of us the winning hand: and whether you feel the Don is a portrait of human frailty, an Enlightenment hero, a poster-boy for class anarchy, a neurotic egotist with profound emotional issues, an overindulged misogynistic patriarchal oppressor (perhaps a little narrow), or just a lucky sod, the fact is that you can take almost any position on him, and yet never feel convinced that your view is either wrong or complete. Consequently, his and Mozart's seduction of us is never finished, and he remains perennially intriguing.

Tim Benjamin has moved part of Mozart's great story forward in Madame X: Masetto (now an impoverished immigrant artist, more reminiscent of Puccini's poet Rodolfo than Mozart's Masetto) and Zerlina (all girlish scruples definitely removed, along with most of her charm) encounter the Don again in the shape of a cardboard-cutout capitalist, Mr Wilmore, whose seduction of Zerlina this time is bald, transactional and unpleasant: and, worst of all, successful - which, sadly, is not an adjective that can be applied to this new opera.

More caricatures than people, Tim Benjamin gives us a collection of such unlovable characters that, despite the best of efforts from both audience and cast, it's hard to connect with them. Not only are they unappealing, they are also fundamentally unrealistic. Take Botney, Masetto's thoroughly unlikeable agent (played very well, nevertheless, by Jon Stainsby, though his sonorous tenor and deft acting cannot rescue the character). Botney speaks solely in aphorisms of exceptional dullness, as if he's trapped inside an endless game of Catchphrase. The effect, unfortunately, is not interesting: it's just insanely irritating, serving constantly to hold up the action, rather than moving it forward. What could be an incorrigibly avaricious, squalidly human character is reduced to a robotic contradiction in terms: Botney describes Zerlina's brutal murder and mutilation, to the freshly-bereaved Masetto, as, "A little spot of trouble." Not even the Kray Twins, one suspects, would be quite so understated. Lady Brannoch (played with gusto and determination by the luminously beautiful Taylor Wilson) is, we are told, a Wildean character: but Oscar Wilde never created a woman with so little depth. Benjamin's is a caricature in the worst sense: not a loving tongue-in-cheek reflection, not a diligent copy, but simply a summary of snotty bad habits without any human relief.

Marc Callahan injects muscular nastiness into Wilmore, singing and acting beautifully, but Callahan's many talents find only a narrow outlet in this paper-thin villain, though he does as much as he can with him. In Botney's resolutely one-dimensional nastiness, Wilmore's unrelieved greediness, and Lady Brannoch's impenetrably immature snobbishness, Benjamin stretches our suspension of disbelief well beyond breaking point: Zerlina's acquiescence in Wilmore's dirty bargain subsequently makes no sense, nor does Masetto's lack of will to prevent it, given that they love each other and have enough money for wine and absinthe (though allegedly not for food?). Even abstract philosophical works like Woyzeck, or symbolic surrealist fantasties like The Cunning Little Vixen and Die Frau ohne Schatten, take care to ensure their internal emotional dynamics, however convoluted or strange on the surface, make some kind of sense. Madame X doesn't.

Benjamin's problems persist beyond his characters into his plot, which is skewed by an unrelieved, unrefined social chippiness straight out of Billy Elliot, reducing everyone's actions to sordid enactments of prejudice which, again, are so unrealistic as to leave us quite cold. The patrons are ignorant and keen to flaunt their money; the artists are poor, hard done by, and exploited; the artists then wreak a subtle revenge which the patrons can't see coming... It is an old, old trope and, treated like this, creaks obviously and unsubtly at every turn. Amid so much bold, exciting, fresh new opera in London, this feels like a hoary throwback, aesthetically and intellectually. The overall intention was a supposed to be reference to Jacobean revenge tragedy: the effect falls far short of that wonderful, bloody, vicious genre. Madame X is so achingly slow that none of the resolute, ruthless intensity of a proper revenge drama is ever achieved. I normally don't notice time passing when I'm watching an opera. With this, I could feel the minutes ticking by.

The one spot of brightness in an otherwise protracted, dull evening is Tom Morss' Masetto. Morss, a gentle and self-effacing presence on stage, did his very best with a fairly limp role, communicating a sense of bewildered humility and inner conviction which gave Masetto artistic intensity and conviction, if not verve: a highly effective, emotive performance. Morss' life was not made easy by Benjamin's decision to allow Masetto only to communicate through the titles of paintings: however, he brought wry and bitter life to all these references. In one beautiful aria, when he was finally allowed a whole text (first set to music by Dowland) with which to express his emotions, Morss sang with a stunning plangency which was well worth staying for, and heralds an exciting ability. Benjamin's restriction of Masetto to those titles, meanwhile, throws up the one interesting question of the night: do artists only communicate through their art, and if a painting is worth a thousand words, should painting be their only form of communication? It also plays upon the much-discussed line between genius and madness, the line between intelligence and autism, and the ironic separation of the artist from the world which he alone can intimately explain. As an idea, it's a good one: but the opera doesn't allow it to get very far, smothered as it is by an exhausted storyline.

The referential tics overflow into the music, which descends into caricature and self-conscious pastiche much of the time, though we do have the momentary comfort of Masetto's lovely grief aria, and some nice harmonies in the ecclesiastical chanting which memorably opens Act II (accompanied only by church bells). Antony Brannick conducts his small and gifted ensemble with delicacy and precision, and their spirited playing rewards his efforts.

I was excited about seeing Madame X, and it could have appealed to me in so many ways: Benjamin's intentions were admirable, some of his ideas interesting, and the programme notes make for a fascinating read which promises much. It takes a lot of courage to poach characters from Mozart; but it is dangerous to do so and not to do them justice. Rather than continuing the ageless seduction, Madame X is very hard to enjoy, despite some wonderful efforts from a strong cast: one suspects Don Giovanni would have tutted, sighed, and smiled enigmatically, saying, "Well - that's all very well - but it's not how I would do it".