Alastair Sim's headmistress in
'The Belles of St Trinian's' (1954)

One of the most memorable lines in Bruce Robinson's Withnail & I (1986) comes when the hapless Withnail confesses to a passing farmer that "we've gone on holiday by mistake". While easy enough to translate, this has proved almost completely baffling to anyone who isn't a native Briton, as the concept itself is so bizarre. But a British audience would find it entirely plausible.

Eccentricity is such a quintessentially British tradition that it's sometimes hard to explain to foreigners quite how embedded the mentality is in the national consciousness. While many nations, either officially or unofficially, shun or actively suppress ideas and behaviour that deviates from the norm, the British celebrate it. Patrick Moore has a distinguished reputation as an astronomer, but this isn't the main reason for him notching up half a century as the longest-serving presenter of a single programme (The Sky At Night, 1957-). Jimmy Savile owes the success and longevity of Jim'll Fix It (1975-94) to his essential otherworldliness, while Richard Massingham's public information films derive their manifold delights from his realisation that the British would be far more receptive to earnest government-sponsored messages if they were delivered in a memorably eccentric fashion.

This principle also holds true for television advertising, the performers often chosen for their existing reputations. Leonard Rossiter is a particularly good example, since his Alan Parker-directed Cinzano ads, which invariably involve him spilling his drink over the long-suffering Joan Collins, drew on prior familiarity with Rossiter's other characters. These include the sleazy landlord Rigsby in Rising Damp (ITV, 1974-78) and the troubled title role in The Fall and Rise of Reginald Perrin (BBC, 1976) and its sequels - the last of these underlining the fine line between eccentricity and genuine mental instability. This theme was also explored by Deric Longden's Lost for Words (ITV, 1999), where Thora Hird's performance as his real-life mother sees her charming scattiness deteriorate into dementia. This was never a fate that befell any of Margaret Rutherford's characters, who were always completely unruffled when confronted with the outwardly outlandish, such as a mermaid in Miranda (1948) or the title character of An Alligator Named Daisy (1955).

We often hear about "English eccentrics", but that has more to do with alliteration than any implication that eccentricity is alien to the Welsh, the Irish or the Scots. The mere existence of Tommy Cooper, Spike Milligan and Alastair Sim give the lie to that, and Sim in particular seems to have been as eccentric in his personal life as he was on screen. His characters seem to inhabit a different plane from those in the rest of the film: his Inspector Cockrill in Green for Danger (1946) adding an unexpected comic edge to a hitherto serious thriller, his serial killer in The Green Man (1956) offering an entirely convincing justification for bumping off the pompous and self-important, and he also carried off a startlingly convincing drag performance as Miss Fritton, "progressive" headmistress of the anarchic St Trinian's girls' school.

Spike Milligan's influence was even more far-reaching, it being possible to trace a direct line from the groundbreaking radio programme The Goon Show through multiple generations of television comedy. This very much includes Monty Python's Flying Circus (1969-74), perhaps the most internationally successful attempt at exporting British eccentricity, unless one counts The Beatles - whose individuality was derived as much from personal eccentricity as from musical prowess.

But the greatest of all British eccentrics is a fictional character, Doctor Who. Originally conceived primarily as an educational series, the programme owes its longevity to a very British form of sci-fi surrealism that achieves much of its effect by being played totally straight - not least by the actors who have played the title character's multiple regenerations. It is no coincidence that the most enduringly popular Doctor, Tom Baker, was also portrayed by the actor perceived as being the most genuinely eccentric in real life, a persona exploited to brilliant effect via his narration of the sketch series Little Britain (2003-05) - itself a celebration of British eccentricity.