Where are political parties born? Labour might say workplaces, the Conservatives their clubs and stately homes, but the Green party – which is 40 years old next week – can be precise: the Bridge Inn at Napton, in Warwickshire, where an unlikely group of lawyers and estate agents used to meet for a drink after work in the early 1970s.

Hardly revolutionary, but it was only when Coventry solicitor Lesley Whittaker passed around a copy of Playboy magazine, which she had bought in WH Smith, that these ultimate middle Englanders resolved to challenge the UK political establishment.

The magazine had an interview with US academic and population scientist Paul Ehrlich, who predicted famine and apocalypse if numbers continued to grow. It scared the hell out of Whittaker and her solicitor husband, Tony. "Good god. The whole thing's going horrible. What are we going to do about it?" she remembered thinking.

Because no one had a clue how to set up a party, they did what seemed right: called a meeting. Disillusioned by UK party politics but inspired by new groups like Friends of the Earth and Greenpeace, they resolved to call themselves PEOPLE and, on 23 February 1973, 43 people, ranging from a probation officer to factory workers and students, went to the party's first meeting in the Whittakers' solicitors' office.

From the start, the members eschewed leaders, and were obsessively, even painfully democratic. But they understood hype: "As chill rains sweep a strike-beleaguered and severely troubled United Kingdom PEOPLE explodes into the public's imagination," someone wrote in the party's first newsletter. The launch was almost universally ignored.

Britain had just joined Europe and North Sea oil was still on the horizon, but soon PEOPLE had a manifesto, and had merged with the miniscule Movement for Survival, set up by Teddy Goldsmith, uncle of present Tory MP Zac Goldsmith. Its first national conference in June 1973 attracted nearly 100 people at £9.50 a head to the Allesley hotel off the A45 in Coventry.

The party was small but had slogans for satirists to die for: "Let us move to a new age. Forget the old. And let the dead yesterdays bury the philosophy of authority and capital gains and communist psychology cults," railed one early speechwriter. "Tomorrow may come. Let us be ready for it," was another.

PEOPLE's utopianism still shocks. In an anonymous paper, probably written by Goldsmith, the author argued that Britain's population must be radically reduced, and business and unions be "restricted" to boost small-scale, community-based concerns. (The author then asked: "Is this just ingenuous wishful thinking?")

"The problem is vast; survival depends on good husbandry of every resource. We do not know any answers, but we've got to think darned hard about the problems. A system like this cannot survive. We expect it to end in our lifetimes," he wrote.

Whittaker said: "We were being fed apocalypse. Yes, we were eccentric. Our views were strange. Our business took a hammering."

Today the Greens admit they still don't know all the answers but claim to be the UK's fourth party, with up to one million supporters in this country, and tens of millions across Europe. But in mid-1973, PEOPLE thought 500,000 people at most might share their views. They hoped 600 people would stand for them in the 1974 election but in the end had only seven candidates, polling about 4,500 votes.

Realising no one had heard of them, they changed their name in 1975. "We considered the Conservation party, but that was too close to the Conservatives. We thought about Environment party but that was too narrow. Green just meant 'wet behind the ears'. So we chose Ecology. It wasn't even in the dictionary," said Whittaker.

But the change brought in a new group of teachers, students and intellectuals, including Jonathon Porritt, Sara Parkin, Derek Wall and the economist Paul Ekins. "There was a lot of literature around then like Small is Beautiful by Shumacher. But Goldsmith's Blueprint for Survival was for me the critical work, the one which pushed the panic button," said Porritt. "There was a great sense of urgency and passion. It's interesting, but pretty much everything that this advance guard was talking about has come true."

Few people expected the Greens to get to their 20th anniversary, let alone 40th. But 300 people will meet on Friday in Nottingham for the spring conference with the new leader Natalie Bennett, their first national MP, Caroline Lucas, two MEPs and more than 100 councillors.

Bennett, who previously edited Guardian Weekly, is confident. "We are in good shape. We're good at democracy. We're national but still local. We are one of the big four but we have a very different model of leadership. There's no hint of dictatorship here. We've got good prospects, are making an impact and we'll do well in the county elections. Just a 1.6% swing will see us triple the number of MEPs next year and then we'll be set for the general election."

But at 40, have the Greens lost their radicalism? "We are just as radical in our way as the people who set up PEOPLE," said Bennett. "You just are where you are."