So begins the final act of the enchanting little play working titled Waiting for Gordtogo, with rumours concerning the Prime Minister's mental wellbeing infusing the internet. Gossip of the kind often afflicts PMs. Margaret Thatcher was regarded as deranged by the late 1980s, even by her admirers, and some concluded that Mr Tony Blair was bananas long before he went off to sprinkle the fairy dust of peace upon the Middle East.

The salient difference between the familiar sense that leaders are in some indistinct, undiagnosed way off their chumps and the concerns about Gordon is that there are unusually specific hints about the latter. Senior Whitehall bods are reported as noting that he was recently given a long list of things he absolutely must avoid, and that among these are Chianti and cheese. Both are well-known for causing a violent, even lethal reaction to a specific group of heavy duty antidepressants known as MAOIs (Monoamine Oxidase Inhibitors).

Fans of Horace Rumpole may recall a story in which the old boy gets a young GP off the charge of murdering his wife, only then discovering that the woman was on just such a drug, and that on the night she died her adoring husband had cooked her a cheese soufflé.

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MAOIs are a long outmoded strain of drug, but might still be prescribed for a patient not responding to the Prozac generation of serotonin inhibitors. In the absence of David Miliband or Alan Johnson arriving for a Cabinet meeting with a wicker-clad bottle of Spanish red and a Pizza Express Quattro Formaggi, there is no obvious way of confirming this, since any opposition or media enquiry would blithely be deflected with a non-denial denial. You'd have thought that whether our Prime Minister is severely clinically depressed falls loosely under the public interest header, but what can you do? Our political system regards secrecy less as desirable than its raison d'etre.

Whether literally the case or not, however, this rumour carries the kind of psychological truth that tends to be more damaging than fact. John Major never tucked his shirt into his knickers, despite the claims of that fierce enemy of trivialising political journalism Alastair Campbell. Nor, so far as we know, was he a martyr to a frozen pea addiction. What mattered was that he seemed the kind of chap who might. So it is with Gordon. He may not be on anything stronger than St John's wort ... but if not, you can't help feeling, he bleeding well ought to be.

In fact, contemplating the near future – and if Joseph Heller were still with us, he could probably coin a catchy phrase for this paradox – the only way he might begin to persuade us of his sanity is by publicly admitting to being on psychiatric medication. For the prospect of another eight months of this is enough to drive the sunniest-natured among us – yes, even Kriss Akabusi – into the most savagely distempered of Churchillian Black Dogs.

The script for the period between now and early summer next year is already written, if not quite set in stone, and it couldn't be gloomier. The next few weeks will be devoted to a revival of speculation about whether his Cabinet will take their final chance to put him out of their misery. It's already starting. Fuelled by Gordon's classically adroit handling of the Libyan bomber's release, whispers of a putsch in the aftermath of a dismal party conference gently mount.

Much as you relish the novelty, what with it being a full three months since the last abortive coup, it is impossible to picture him going now. Deftly transferring the medical analogising from the mental to the physical, Labour is the terminal patient offered radical surgery that at best will slightly improve the quality of life, but cannot save it. With a few months left, who needs the additional agony, not to mention the risk that the operation might be fatal itself? Less traumatic, surely, to up the diamorphine dosage, float off on the fantasy of a last minute miracle cure, and eke out whatever crumbs of pleasure are still to be had.

By the middle of October, that last window will have closed, and the party will grudgingly accept that it's stuck with Jolly Jack Tar at the helm. At that point, two wars will begin ... the phoney electoral battle, already decided, against the Tories, and the real one for Labour's future in opposition between the various factions within. Even the Baron Mandelson, noblest and cleverest of his generation, will find it beyond his cocktail of charm and menace to keep the lid on that, and should turn in earnest to fixing the succession for his preferred candidate.

Six months of internecine skirmishing between the rump armies of puritan Brownites and cavalier Blairites will ensue, while Gordon pursues his own fight (also lost) with the increasingly independent Alistair Darling over how frank to be with us about the extent of hardship ahead. Both will rejoice to tell us that the recession is over, but with the rise in employment lagging miles behind any recovery this will do them little good. In fact, it will do them harm. If the economy is back in growth, or about to be, they lose their most effective argument against voting in those novice Tories.

What motivation remains to hold on to such a gruesome nurse (Ratched from One Flew Over The Cuckoo's Nest isn't in it) if there is nothing worse in view? The economy was booming in 1997, and much electoral joy that brought Mr Major.

And then, possibly after an anaemic pastiche of a US televised presidential debate, it ends in May or June... not with a bang but with a whimper of despair. What is uniquely depressing here is that the prospect of being rid of an exhausted, discredited, useless and despised government offers not one iota of excitement, enthusiasm or optimism.

In 1964, after 13 years of the Tories, Harold Wilson was powered into office, albeit by a whisker, by his white heat of technology. In 1979 even those who loathed and feared Mrs Thatcher were a little thrilled by the energy she promised to bring to annihilating the post-war consensus. Eighteen years later, Mr Tony Blair's new dawn momentarily warmed even those of us who had grave doubts.

This time round, there isn't even a pretence that Mr Cameron is what our former possession across the Atlantic knows as an agent of change. While Obama fights his dramatic rearguard to salvage universal health care for Americans, the Conservative leader strives to save thruppence ha'penny by cutting subsidies in parliamentary canteens. Like a boxer with a huge points lead as the bell for the final round clangs, all he has to do is coast and hide his chin. He knows he need do nothing to win well, and doing nothing other than affect an aura of paternalistic competence will, we must assume, be the hallmark of his administration.

Eight months of this, then, followed by eight years of that. If Gordon isn't on the pills, he's an even bigger meshuggenah than we thought. The only worthwhile question left for him to answer is whether he's stockpiled enough to spare a few for the rest of us.