In his latest, coruscating outsider’s examination of our ruling classes, Sir David Hare (Lancing; Jesus College, Cambridge; the National Theatre) looks at the business of donations to political parties. Though Hare has claimed that the play is “pure fiction”, there are some remarkable resemblances to reality.

Otto Fallon (Stanley Townsend) is a pop-music multimillionaire and fundraiser for new Labour. When the daughter of the home secretary, Meredith Guest (Tamsin Greig), is threatened with expulsion from school after a minor drugs bust, Fallon steps in and buys the school a placatory new gym. But the daughter, Suzette, only just 16, has got drunk at a party and had sex with four men in a row. One of them was a filthy journalist, and she may have said something to him.

This sensationalist and unlikely scenario is only made worse by Hare’s astonishingly crude characterisation. His portrait of Lord Levy - oops, Otto Fallon - drips the kind of leftist Hampstead snobbery Mrs Thatcher used to attract from people such as that sage and polymath Dr Jonathan Miller. Fallon is supposed to appear vulgar, but really it’s Hare’s conception, his dramaturgy, that is terminally vulgar. Fallon has a substantial girth and a stubby iron-grey ponytail, and actually says he was educated in the “university of life”. Strike a light, guv’nor! And having him declare, in a thick north London Jewish accent, that “Money’s the thing” - well, perhaps that is quite brave in today’s climate, I’m not sure.

For his latterday saint, Hare has a former state-school music teacher, Lori Drysdale, who now busks on the Underground. Nicola Walker tries to bring her to life, but there’s about as much chance of reanimating one of Dr Gunther von Hagens’s plasticised corpses.

Even the marvellous Greig, stalking around in her black suit and stilettos - nobody stalks like Greig - can’t do much. The single, glorious exception in this valiant but lifeless cast is Pip Carter as Frank Pegg, Fallon’s gaunt, unsmiling factotum, who creates a fantastically funny performance from only a few lines. Otherwise, Gethsemane is a portentously titled, three-hour state-of-the-nation homily, composed of nothing but meandering dialogue and stilted pseudo-wit. Greig has to deliver lines like “It’s an organised hypocrisy and it’s called democracy” in all seriousness, as if it’s a political aperçu worthy of Walter Bagehot. Even worse, it’s verse. Can’t Hare hear how comically it rhymes?

So far, so bad. Then Geoff Benzine appears (“Benzine” as in toxic and carcinogenic). He is the filthy journalist who had sex with Suzette only weeks after her 16th birthday. To prove that his view of journalists is unarguable, Preacher Hare outs him as a borderline paedophile rapist: the trashiest, laziest way of characterising a villain. I found myself snirtling helplessly. (To snirtle: to try to suppress one’s laughter, a neglected but useful word, especially during Hare plays.)

I snirtled again at the last scene, in which Drysdale - a former state-school teacher, you may recall - is holidaying with Suzette in a “small hotel” that “gives out onto the blue Sicilian sea”. Maybe it comes as a revelation to Sir David Hare, the passionate socialist and people’s playwright, but outside the purlieus of his comfortably upholstered world, not many state-school teachers - my sisters, for instance - holiday in “small hotels” with sea views over the exclusive Italian Med.

There is no inadvertent humour, alas, to brighten up an interminable scene between Meredith Guest and the PM, Alec Beasley, who wears jeans and plays the drums. Yes, he is very like Tony Blair. Hare may deny these are portraits, may deny that Guest is very like Tessa Jowell, for instance - but why have the National’s lawyers had to check the text for libel? Gethsemane aims to skewer the media for being intrusive and cynical while putting real people on stage, poorly disguised, then spotlighting their careers, hobbies, mannerisms, even their marital difficulties, with an intrusiveness that exceeds that of the worst tabloid. On top of that, it claims the moral high ground. For Hare to claim this work is “pure fiction” is breathtakingly disingenuous.

“There is more independence of thought in the Finchley Rotary Club than there is in the British Establishment,” Fallon says at one point. Quite so. And Hare is at the very heart of that Establishment. Compare his latest work with Harley Granville Barker’smagnificent Waste, at the Almeidarecently, or with Trollope’s Palliser novels, or even The West Wing, and you see just how thin and sour is his political imagination.

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