Smit is a former pop group manager in his mid forties who found a new outlet for his entrepreneurial skills when, a decade ago, he took on the thousand-acre grounds of a derelict Victorian estate at nearby Megavissey, and restored them as the Lost Gardens of Heligan. What with the book and the TV series (Smit has never been one to hide his light under a bushel), Heligan became rapidly famous and is now the most-visited private garden in England. Eden is a lot smaller in area - a mere 37 acres or so, though you'd never think so to look at it. But its sense of scale is from another planet. The conservatory complex is 1476 feet long, 360 feet at its widest point, 180 feet high at its tallest. Though measurement is a little difficult, since the organic form of the glasshouses is the antithesis of your normal, rectilinear building, and the fact that they cling to the slope of the cliffs - so different from the essentially flat floors of most rival glasshouses - makes definitions of height somewhat meaningless. They face south - the sea is just visible at the end of a valley a few miles away - making this the biggest lean-to in the world. It makes Kew’s famous Tropical and Temperate Houses - an inspiration to a very British strain of architects, epitomised by Grimshaw - look like potting sheds. It even trounces Lord (Norman) Foster's Great Glasshouse in the new National Botanic Garden of Wales near Carmarthen, a classically lovely but lower-tech and less awe-inspiring solution.

Eden seemed at first like pub talk, beermat stuff, the sort of idea that you’d normally dismiss as mere braggadocio. But Smit, it becomes very clear when you meet him, is a restless, fidgety fellow, disinclined to sit back on his laurels. He’d done the music business, he’d done the big restoration project, now it was time for something new. His sidekicks at Heligan suggested that, having re-established the peculiarly Cornish character of those gardens and the history of those who tended them before the Great War, what about telling the story of the plants from the rest of the world? The ones we all depend on for survival, since without plants we’d all be dead?

Smit, who must have a strong streak of explorational zeal and trading nous in his Anglo-Dutch genes, went for it in his characteristic bull-in-a-chinashop way. The idea was refined down a little, since to display all the plants in the world you would need most of the world anyway. They decided that a place to display around 4000 species was needed, perhaps built somewhere in the bizarre industry-scarred landscape of the china clay country spreading north from nearby St. Austell. The idea meant regeneration, visitors, jobs, in an area of high unemployment - but which, unlike the differently wrecked post-industrial north, is also a place that people like to visit, especially in the summer. Heligan, like the Tate St. Ives, had proved that millions would come to a new year-round attraction here. But Eden would be truly all-weather. It was a wizard wheeze, well over the top, and that was where it might have rested had not Britain decided to run a National Lottery and set up a Millennium Commission to distribute some of its proceeds. It was 1994.

Quick as ever to spot an opportunity, Smit decided to act. He phoned a local Cornish architect with national contacts, Jonathan Ball. Ball, sketched out a possible scheme, for a flat site, to get things moving. Then, with admirable selflessness, he said that for a project this ambitious, you needed a top-name, internationally acclaimed architect. He recommended Nicholas Grimshaw and Partners, on the strength of their recently-completed Waterloo International station in London. This organic, sinuously snaking glass palace for Eurostar trains showed that Grimshaw was fully aware of the long British tradition of the advanced glasshouse - and could bring the old idea right up to date.