I suppose you could call Boris Johnson either a political celebrity — as Sonia Purnell does in her splendid new biography of the London Mayor — or a celebrity politician. Are they the same thing? Some expert on semantics please advise.

Anyway it’s the celebrity element that’s important. As Ms Purnell demonstrates, Boris has spent much of his life in pursuit of fame — “that last infirmity of noble mind,” as John Milton put it in a poem, Lycidas, that the well-educated Mr Johnson knows by heart (as I do).

His fame is what made him electable, both as MP for Henley and later in London. It’s a trick that usually works. I noted this nearly 40 years in first-hand observation of the Isle of Ely election of 1973 in which Clement Freud — well-known like Boris as a panel game ‘character’ — triumphed for the liberals, to the surprise of some.

If Sonia’s book — I feel she will permit the first name — can be said to make one principal point it is that her subject is a very focused gentleman. A financial journalist, she had an opportunity for close-up study of Johnson when they worked together in the early 1990s in the Daily Telegraph’s Brussels bureau.

She says that his shambolic approach, like the carefully tousled hair, is a clever disguise assumed by someone whose eye is always on the main chance. His aim, the author believes, is nothing less than the prime ministership, though his slightly younger Eton contemporary, David Cameron, might have other ideas about that.

There, I’ve used it — the ‘E’ word. No article concerning Johnson (or indeed Cameron) leaves it out for long.

I was mildly surprised to discover from Just Boris (Aurum Press, £20) that he is not the latest in a long line of Etonians. In fact his dad, Stanley, didn’t go there but to the much-less-nobby Sherborne.

Once there, however, Boris befriended some of the smartest people, including the Princess of Wales’s brother, Viscount Althorp, and Darius Guppy, later to embroil him in trouble when he tried to call in the services of his pal to supply the address of a journalist he was minded to have roughed up.

The redoubtable Mrs Julia Budworth, who tried to shift Boris’s increasingly famous sister Rachel from the editor’s chair at The Lady, may have had a point when she called her “a [social] climber, like the other Johnsons”.

Boris also appears to be — or to have been — a serial adulterer. Sonia names the names, and then points out how lucky he has been in Marina (barrister daughter of the journalist Charles Wheeler), his second wife and mother of his four children.

“Marina’s dignified silence has helped Boris to sail through each new set of revelations virtually unscathed — both must know that if she turned against him in public or divorced him, his career might never recover.

“He has also been fortunate in his mistresses — none has yet denounced him.” One can’t help noticing that ‘yet’.

While there is much in this long (433 pages) and exceptionally well-written biography that will annoy its subject, there is much, too, that will please him.

Instance: “Boris has been endowed with the brains, the emotional intelligence, self-belief and perhaps most of all good luck, to make it. He is blessed with immense charisma, wit, sex appeal and celebrity gold dust; he is also recognised and loved by millions.”

I don’t call that a hatchet job.