Four weeks ago, Clark launched a weekly Thursday-night event at Heaven, a nightclub under Charing Cross station. Called Megatripolis, it falls some way short of the original blueprint, but there is an unmistakable Utopian idealism that attracts large numbers of young people. Hundreds are turned away, but the 1,500 inside are confronted with a mass of information and choice: psychedelic paraphernalia and pamphlets, massage booths, esoteric bookstalls, kaleidoscopic TV installations, ambient music and guest lecturers (last week's was Francis Huxley, nephew of Aldous). Megatripolitans can dance, sit, talk, listen, play music, doodle on computers, or just 'soak up the vibe'.
Clark is 50. He is that rare type, the idealist who has cradled his dream, biding his time, awaiting planetary conjunctions, and then acted upon it. Early on he decided to live outside convention, and his life has been one long, strange trip.
'The demi-monde always appealed to me as a career option,' he says in a time-worn Glaswegian brogue. He is sitting by the window, in the 'psychic power spot' of his study, peering out over West Hampstead, stroking his goatee beard. Nearby, a paper rainbow has been pasted to the wall. 'I was about 14, looking at the limited number of social roles that were available. There were no rock stars then, but the bohemians and artists seemed to be having the best fun, according to my mother's magazines, like Woman's Own.'
The son of a gentlemanly lawyer who 'would never dream of getting his hands dirty', Clark studied psychology at Glasgow University and became a beatnik, 'probably the only one in the city'. By 1962, he recalls, he had hair halfway down his back and was going barefoot to college, even when it snowed.
Holidays were spent on London's beatnik cafe circuit, then based mainly in Hampstead, and on Jersey, where an enclave of beats would descend for the summer. 'They were all called things like Manchester Mike, Bongo Willie and stuff. Everybody would have one fantastic item of clothing, usually a jacket, and they'd wear it day and night, sleep in it. We were crashing out in parks, panhandling in the street, fishing half-eaten sandwiches out of litter bins, and signing on each morning for one day's food allowance.' Eventually, the entire gang was deported from the Channel Isles for sleeping 20 in a two-bed chalet.
Clark left university in 1965 with an honours degree. He was eager to see the world. He headed for Formentera, off Ibiza, which sounded 'happening'. Here, he first encountered 'hipsters', the American beats who bridged beatnik and hippy, and on their advice took his first acid trip, which he bought legally over the counter.
'We thought acid was the answer to all society's ills,' says Clark, who is disarmingly frank about his generation's naivety. 'You just watched one person after another trying it, having a blissful experience and becoming beautiful, sensitive, loving people.' It seemed just a matter of time 'until the President would try it, and that would be the end of all problems'.
As well as savouring the first flowering of free love ('there were lots of Scandinavian goddesses around,' he chuckles), Clark was deeply impressed by hipsters. 'They all had these little black moustaches and carried the three or four almost sacred texts: the I-ching, the Tibetan Book of the Dead, the Psychedelic Review, and a Hesse novel - usually Siddhartha. 'They were the four things you had to read in Formentera.' He had a glowing realisation that this was 'an alternative society that was international, not just working-class kids from Liverpool with bare feet and baggy jerseys'.
Clark returned to 'swinging' London for long enough to see it wasn't to his taste then went searching for a civilisation that could resist the 'concrete wave covering the planet'. He spent the best part of 10 years looking - in Africa, South America, the States, especially California, and Canada. After resisting India for years, he finally relented; as expected, it was the trip to end all trips. 'There was nowhere more exotic left after that,' he says with a smile. 'And you also had to come back and try to reintegrate into the West.'
For a decade and a half he had been living versions of the hippy ideal, but by the mid-Seventies he felt that the true ideal had been sold out. For this he attaches a lot of blame to David Bowie, a culture hero gone wrong. Hippy ideals forced pop stars to dress down, to look like everybody else: Bowie put them back on a pedestal. Clark spent the last half of the Seventies abroad, travelling and living in India, Ibiza and California. Subsequently, his first contact with punk came when he entered a London party in 1980 to be greeted with the cry 'Oi] No fuckin' hippies]' from a spikey head in the corner. Clark was shocked. He was used to hostility from police and 'straights', but this was his first encounter with the Malcolm McLaren-inspired generation gap, and its slogan: 'Never trust a hippy'.
The next five years, he says, were a time of deep depression. He stopped taking LSD, which until then he had used 'almost as a sacrament' every three months, discarded his trusted I-Ching, and went into 'a kind of hibernation' until 1985, when he suddenly started to get the feeling that the time was ripe for change. He started a small fanzine called the Encyclopaedia Psychedelica, a slapdash collection of original and photocopied articles on hippyish topics, hoping to regenerate enthusiasm for the hippy philosophy. Immediately, he received letters from all over the country, confirming his intuition.
Clark persevered with his almost single-handed effort to generate an outbreak of good consciousness among the youth. Unbeknown to him, it was actually happening. It took a couple of young designers who walked into his office and told him about the rave scene and what he now thinks of as 'the summer of love, 1987'.
'It was clearly the Next Wave, the thing I'd been talking about, there was no doubt about it. The key question was: why would Thatcherite kids, on the way up, choose to dress down? They didn't quite understand why they were doing it. It was just a fashion thing for them. But it was very definitely a sociological shift, they were beginning to drop out and reject the whole Thatcherite philosophy. Only they didn't realise it then.
'The response of Thatcher's government to the rave scene was a colossal mistake. She'd raised them on the entrepreneurial spirit, and here were these raves run by model capitalists, charging pounds25 a head - some people made a quarter of a million pounds in one night. She should have been giving them Thatcher Awards for Enterprise. Instead she banned the whole thing, set the dogs on them, and they couldn't understand it. And that really radicalised the whole thing, sent it off on its present course.
'The difference between the Sixties and now is that then you dropped out. Now, you drop into an alternative world, you're no longer alone, or even in a minority, you can enter this kind of demi-monde, where you can cast about for like-minded souls and a personally tailored reality.'
Indeed. There is a rapidly expanding and increasingly radicalised demi-monde, guided by ageing hippies and ex-straights turned zippy ('Zen-inspired pagan professionals'). But the momentum is fuelled by a new, disaffected generation of people under 30 with little or no desire to play minor supporting roles in a crumbling, outdated form of free-market capitalism.
'They are starting to find they have everything they want and need,' Clark says of his constituency. 'Why should they work to support a system that cannot even begin to comprehend them and the things that they believe in?'
And he sits there grinning broadly, his legs crossed, the toe of his slipper held together with Gaffa tape. It is a strange image of triumph, but somehow fitting. Does he feel that his life, lived on the edges of society, has now been vindicated?
'Oh yeah. I went through the wilderness years, yeah.' And his face crumples into a sly smile.