At the time, it was - at pounds 120,000 per episode - the world's most expensive television programme. Although only 17 episodes were filmed, endless re- runs have reaffirmed its cult status (the show is currently being screened by Channel 4 on Wednesday nights).
The Prisoner tells the perplexing story of Number Six, a former intelligence agent who, after resigning, finds himself spirited away to an island known only as the Village from where there is no escape. Its bizarre inhabitants are largely well-known British character actors such as Donald Sinden, Patrick Cargill, Paul Eddington and Leo McKern.
Like all cults, The Prisoner boasts its obsessive disciples. The University of California at Berkeley offers a degree in television studies focusing almost exclusively on the series, and the Portmeirion Hotel in North Wales, where it was filmed, has been overrun with Japanese visitors since the programme was shown there last summer.
Items on sale today at the Great Western Hotel in Praed Street, Paddington, range from videos to replicas of Patrick McGoohan's Lotus 7 car at about pounds 18,000.
There is a world of difference between Prisoner fans and those of other cult television series such as Dr Who and Star Trek, according to James Roberts of Six of One, the official Prisoner Appreciation Society. Mr Roberts, 23, a trainee barrister and the convention co-ordinator, was not born when the programme was first screened in 1968. He believes the show's appeal lies in its complexity and the frustration it engenders in viewers.
'Although Prisoner fans come from a range of backgrounds, we've got a lot who are rather highbrow - doctors, lawyers, lecturers, that sort of thing - and plenty of journalists and television people,' he said.
The suggestion that people attending last year's convention should wear Prisoner clothes did not go down too well with some fans, who felt this would be contrary to the ethos of individualism preached by Number Six.
'They really want to sit around and discuss the moral and ethical questions raised by the series, rather than dress up as Cybermen as the Dr Who fans prefer,' said Mr Roberts.
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