“BRITISH politics is totally amateur. That’s why it’s so sexy and toxic.” Such – for what it’s worth – is the view of Sasha Swire, author of the most talked about book of the year. So much has already been said about Diary of an MP’s Wife, it feels as if it’s been out forever, when in fact it is published tomorrow.

This breathtakingly indiscreet account of the wannabees and hangers-on who cluster around No10 like bluebottles swarming over roadkill, roughly spans the ministerial career of Swire’s husband, Sir Hugo, former MP for East Devon, and one of David Cameron’s cronies.

And what a bunch of buffoons they are. The unflattering picture of the ruling elite that emerges from Swire could come from a Restoration comedy, or a Brian Rix farce, with drunken players dashing in and out of each other’s beds, disappearing naked through one door only to emerge from another moments later, adjusting the knot on their ties.

Photographed for a pre-publication interview, Swire had placed a copy of Alan Clark’s diaries prominently by her bedside. The hubris of the association is telling, but in one respect it was an appropriate prop. Conservatives from old school backgrounds seem to have a monopoly on the art of rackety diarising.

As a breed, political diarists are notoriously unreliable witnesses, and are utterly self-serving. Swire fits into a long tradition of backstabbing, self-aggrandizing upper-class scuttlebutts. Her particular talent is in taking scalps, and she adds trophies to her collection with such vigour it’s as if she’s being paid by the kilo.

Alan Clark, the late Tory MP and junior minister, was one of the finest diary writers in modern times. In another league from Swire, his account of serving in Thatcher’s cabinet was sharply observed and incorrigibly bitchy. But unlike Swire, the human wrecking ball, he did much more than spill beans and trash reputations. He had the natural diary keeper’s interest in preserving his experiences, capturing the details that distinguish one person’s life from all others.

The most memorable of his entries did not concern Westminster. Rather, it describes shooting a heron he found stealing fish from his pond. A horrible episode, it shows him taking aim at a magnificent bird, whose botched killing leaves him sobbing: “I cursed and blubbed up in my bedroom, as I changed into jeans and a T-shirt. I was near a nervous breakdown. Yet if it had been a burglar or a vandal I wouldn’t have given a toss. It’s human beings that are the vermin.”

Only a Tory could have written that last line, even if a socialist or liberal might privately have felt the same. It is the willingness to state the unthinkable, to record the unsayable, without hoping or intending to shock but simply as a matter of fact, that sets the great diarists apart.

Among those of this calibre are Harold Nicolson, husband of Vita-Sackville West, and the scurrilous and snobbish Chips Channon, a friend of King Edward VIII and the Duke and Duchess of Windsor. In one entry Channon records that while sleeping in a stately home, in the bed used by George V and Edward VII, he had the misfortune to smash the royal chamber pot. This was not his first such offence.

“At Mentmore once, staying with the Roseberys, I broke Napoleon’s pot in similar circumstance, a very grand affair covered with ‘N’s and Bees.” Although this entry is inoffensive, except perhaps to antique dealers, his work was so sensational it has yet to be published in its full original state.

It has to be said that the Tories manage to create their own class of scandal. The lordly sense of entitlement of the old Etonians who dominate the party seems to attract gossip and treachery, of which Swire’s book is merely the latest example. I suspect many of those she shows in a blokeish and boorish light – Cameron and Osborne particularly – rather relish being the centre of attention, no matter how toe-curling the revelations. How else could Swire have expected to get away with merciless swipes at her own set?

Some, of course, will have already banished her to social Soay. Others will secretly applaud the way she has dished the dirt on their rivals and continue to tolerate her, viper though she has proved herself to be.

I wonder why Scotland has so few kiss-and-tell journals. With the notable exception of the salacious, lusty and penitential James Boswell, we have a dearth of devastatingly honest or vengeful diarists. Naomi Mitchison is the most confessional and unfettered I can think of, but her entries are personal, unshowy and often introspective.

Towards the end of World War Two, on April 23, 1945, she went to give blood: “My fellow blood transfusers talking about prison camps. That really seems to have got under the skin of even Carradale [her farm]. I keep on saying that when some of us talked about concentration camps three years before the war the people who talk about them now wouldn’t listen. One just can’t quite imagine the quality of hell it must be in Berlin. I suppose Hitler and Goering will either get themselves killed or commit suicide. I hope they won’t be martyrs anyhow!”

Since Mitchison, like Boswell, was from the landed classes, perhaps high social status is a common factor in this two-edged literary art. The poet William Soutar stands out as a rare example of a working-class person who kept a remarkable record. Diaries of a Dying Man, his account of the last 13 bedridden years of his life, was hailed by Harold Nicolson as a “brave and animating book”.

Both adjectives could equally be applied to Swire’s eruptions, albeit not in a good way. And although her savagery provides voyeuristic entertainment, it lifts the lid on the manner in which a ruthless clique conducts itself. Books like this can destroy reputations, friendships and careers. In the end, however, they tell us nothing we didn’t already know. And while appearing to be artless, they are as dastardly as those they seek to destroy.

Soutar might have had Swire in mind when he wrote, “A diary is an assassin’s cloak which we wear when we stab a comrade in the back with a pen.”