DJ Taylor: Alastair Lark has devoted the better part of his life to practical jokes

(Mark Long)

Most families have their catchphrases – signature remarks brought out to raise a smile at inter-generational gatherings and social events. The Larks', curiously enough, is, "Headmaster, we're not dealing with normal boys."

This, it turns out, was the judgement pronounced over the retreating figure of Alastair Lark by the deputy headmaster of Ware County Grammar School on the day that, with half-a-dozen fellow conspirators, he was expelled from this establishment for rigging up a rocket-launcher that sent a javelin winging over three playing fields to land in the school swimming pool shortly after two dozen 11-year-olds had clambered out of it.

Alastair, a juvenile-looking 53-year-old with quite a decent job in marine insurance, has devoted the better part of his life to practical jokes of this sort. The pool incident was the merest preliminary in a catalogue of spoofs, wheezes and cocking of snooks at hidebound authority. At the Oxford college to which he finally gained admittance, on the strength of his "distinct if wayward intelligence", he was famous for flooding the junior common room with fake May Ball tickets and printing a bogus number of the college magazine that hinted that the Warden had been arrested on drugs charges.

The world of work put something of a brake on these diversions, but you can't keep a good gagmeister down, and Alastair's early years at Messrs Tender & Mainprice, chartered accountants, were marked by a succession of doctored noticeboards, spurious memoranda, and an incident in which the lavatory in the corner of the executive lounge exploded at the precise moment the firm's senior partner went to use it. Only the other week he is said to have telephoned his own managing director, put on a funny voice and pretended to be a reporter from The Economist.

As for what the Larks make of all this, Alastair's children think he is "a scream"; Mrs Lark is indulgently resigned; his mother-in-law, under whose dinner plate an indoor firework went off just as she was carving the Christmas turkey, has her own views.

If detected in these outrages, Alastair will always apologise with the utmost sincerity and offer the excuse first minted for the deputy headmaster – that it "seemed a good idea at the time".