You can do what you like to Jephtha, Handel’s last complete work — impose any premise you want, but it has an indestructible core. It is an unimproveably dramatic one, too: God has just played a cute trick on the bumptious general who promised to sacrifice the first living thing he sees in exchange for victory over the foul Ammonites, so naturally it’s no stray goat that crosses his path — but his own daughter. What follows is probably the most intense passage in any of Handel’s works, where Jephtha (and Handel with him) struggles to come to terms with the horror he has unleashed.

Clearly, we are in Iphigeneia territory here, signalled by naming the girl (anonymous in the Bible) Iphis. However strong those Greek stories are, the minute you switch Jove for Jehovah, things do get a lot edgier: you expect slightly less capricious standards even from the Old Testament God than those funky Olympians. Given that the 18th century was probably not as practised as we are at separating the crazed old avenger from their Christian God, Thomas Morell’s script is rather daring in its implication, however tactfully diluted, that the Deity is being a bit of a pill.

These days directors tend to give the God thing a swerve and take it out on old Jephtha, and he undeniably comes over bullish and rash, although evidently a pretty useful general, given his ability to whistle up a bunch of shock troops at crucial moments in the form of cherubim and seraphim. There is a decent dramatic justification for making him not very nice: we are forced to feel sorry for the dreadful fellow, even when he’s brought it all on himself with that presumptuous palliness with the heavenly powers.

Director Oliver Mears goes further, taking the God-on-our-side stuff to the limit with Jephtha now heading up a ghastly gang of fundamentalists — and no jolly and wholesome Witness Amish types either. It’s all much more Handmaid’s Tale, peevish puritans, irked by the party animals next door, joylessly chucking paintings and gilt-and-plush furniture on their bonfire. This war we hear about seems to be largely imaginary, consisting of the massacre (titterishly greeted) of those roistering neighbours, themselves a provocatively fluffy version of the text’s occupying-Nazi-style Ammonites.

The thing is … well, what is all this for? Now that theatre directors are universally acknowledged to be the ethics gurus de nos jours, and this gang of monochrome dullards (reflected in a relentlessly grey stage picture, with its louring walls inscribed with grim Book of Judges drivel) dutifully doing a lot of sitting on their prim little benches, are clearly intended not for dramatic or entertaining purposes, you wait at least for the intended pi-jaw: who, exactly, is being insulted or satirised here, and how does it concern us? Mears has got a tad carried away by his premise, trying to shoehorn a sermon about fanaticism and intolerance (not wholly irrelevant, for sure) into something that is really overwhelmingly concerned with central dilemmas about reconciling faith and moral structure with a pitiless universe.

Handel’s oratorios are more concerned with community than his operas, effected by an earnest series of choruses that elaborate the challenges under discussion, but the impact is diluted, to say the least, when they are sung by people we’ve just been instructed to disapprove of. This is the central weakness of this staging: music and action are working against each other — and not in the fruitful way that can happen in the hands of a director like Calixto Bieito, who would chuck out the whole scenario and create a challenging parallel narrative on the stage. Mears’ vision falls between stools, trying out a half-cooked critique that just leaves us hanging.

The crisis comes exactly at Jephtha’s pivot, when Iphis comes tripping out to greet her victorious daddy. Handel finally gets into character and hits us with 40 minutes of music of such massive, unshowy impact that directorial concepts fly out of the window: humans under immense pressure, caught in the jaws of the ineluctable. Handel — old, sick, half-blind, thinking about God and his own mortality — found it immensely difficult to write, notably the culminating chorus. “How dark, O Lord, are thy decrees” ends with a desperate cry of “Whatever is — is right”, repeated nine times as these poor saps try to convince themselves of this dreadful (but in the circumstances, the only available) answer to the zugzwang engineered by fate. It’s the old wrestling match between morally sensate humans and a malevolent universe, expressed with terrific power and economy. The suggestion that Jephtha could somehow “make it go away” by being less of an arsehole is the crassest of cop-outs.

This is that core: Handel sucks you into the spiritual turmoil and won’t let you go — and at the same time, Mears effectively gives up directing. Well, you can get away with that with Handel, and in truth it’s a bit of a relief. Suddenly everything is irrelevant beside the vividly expressed anguish of Jephtha, his wife, Iphis and her boyfriend Hamor, with that chorus trying to articulate the inexpressible. At last, too, the orchestra and chorus, amazingly weak and ropey to this point, dig into the strength of what Handel wrote. With the stage design and movement suddenly set to an evocative stateliness, it becomes very powerful.

That doesn’t last. Iphis’s sentence gets commuted to life in a nunnery, and we’re launched into an imposed wish-fulfilment girl-power finale evidently inserted merely to display the director’s bien-pensant credentials. Eccentrically, it’s also designed — with the chorus standing in the aisles — to block the view from a good number of 200 quid seats, but I don’t think we missed anything important.

You’d hope for something more ambitious to match the composer’s troubled vision

Don’t get me wrong: those 40 minutes are worth a lot, and there are some strong performances. Allan Clayton is a great singer, his Jephtha unremittingly impressive, his moral collapse powerfully conveyed. The blessing — a sudden irruption in the musical darkness of a beautiful, lulling aura of high violins and the tenor’s lovely flowing line — of the despairing-yet-hopeful “Waft, her, angels, through the skies … ” conjuring a completely sincere divine comfort. Jennifer France is a lovely Iphis, very effective in both her radiant acceptance and the strikingly passionate love Handel gives her with Hamor (Cameron Shahbazi). Alice Coote, as the fiery mother outraged by the pronounced sentence, is intensely dramatic but needs a bit more help from a largely anodyne orchestra, surprisingly unidiomatic under conductor Laurence Cummings. Boy treble Kaelan O’Sullivan was fearless in his testing appearance as the angel-ex-machina.

OK, Mears is trying to be serious, though you’d think he could come up with more immediate exemplars of extremism and intolerance than old Ian Paisley, whom he name-shamed in a pre-performance interview (Jephtha gets a very Orange Order sash at one point, too). Though Jephtha is certainly a solemn piece, a bit more joy and light in the first act (very much present in the music but not on stage) would make the awfulness of what follows more effective. Some of Simon Lima Holdsworth’s stage pictures work well, starkly lit by Fabiana Piccioli. It’s not a disaster. Stagings of Jephtha are passing rare, though; we’ve waited a long time for this, and you’d hope for something more ambitious to match the composer’s profoundly troubled, questioning vision. This is a much shallower, more banal excavation, and really it’s only about half as good as what Handel actually wrote.