Cross the bridge to Kent's Lullingstone Castle, a tiny humpbacked road over a shallow river, and you're right back in the hot summer of 1976. It's a miraculous sight: families sitting all along the riverbank while small children play in the water with nets, lying down in the little rapids. (Kids swimming in a river? Is that even legal in 2009?)
Here, in a spectacular setting, with a Tudor gatehouse guarding the residence from prying eyes, Tom Hart Dyke runs his World Garden. Inside the estate's old stone walls, on a single acre, Hart Dyke has collected 8,000 species of plants from all over the world. Yet, for someone who's done all that travelling (he was even taken hostage for nine months) and who has had a BBC2 series made about him, he is extraordinarily unworldly – it's only this year that he has removed his own mobile-phone number from the entrance signs.
So how did he come up with the idea for the World Garden? "I'm a plant nut," he explains. "A man holds a gun to your head, tells you he is going to kill you, then fires off a few rounds; that's when I started to make the plan."
But he is more than a collector; many polythene tunnels, used as greenhouses, testify to his patience as a propagator, filled with hundreds of species grown from seed.
The design of the garden is now complete, each continent having its own area; the world's biggest dahlia dominates the South American bed while Verbesina microptera, a 10ft member of the daisy family, towers over the Mexican plants.
The garden is run on the tiniest budget; much of the work is done by just Hart Dyke and a volunteer, Jim Buttress. Sunflowers weave into sweetcorn, tomatoes grow amid the cosmos, and there's an extraordinary sense of late-summer bounty. Tall grasses and rare South American beauties are just getting into their stride, but there's much still to do: "I'll be out working until the moon comes out," Hart Dyke laughs.
The treat is to go around the garden with Hart Dyke himself; he aims to be on hand every day the gardens open. In early October, there will be a chance for a whole afternoon with him and Mike Nelhams, the celebrated curator of Cornwall's Tresco Abbey Gardens, including an all-you-can-eat tea in the house. For £30, this sounds a superb treat.
There are plants I've never heard of, rare eucalyptuses and unusual agaves from deserts thousands of miles away, in a perfect English setting. And driving away from Lullingstone, past the children swimming in the river, you won't want to leave at all.
A specialist World Garden plant fair is being held at Lullingstone Castle today. See www.lullingstonecastle.co.uk for details
Lullingstone lovelies: Tom Hart Dyke specials
Bishop of Llandaff
This sterling specimen wins fans even from dahlia haters, for its deep-red flowers and bronze foliage. Dahlias need a bit of care through the spring to save them from slugs but get them to a foot tall and they should romp away.
£6.99 a plant, www.crocus.co.uk
Salvia 'Hot Lips'
A hit at 2009 flower shows for its rude red-lipstick- marked flowers. Hardy even in deepest Kent, it's easy to grow. Hart Dyke recommends a hard prune in April.
£1.99 a plant, www.nicholsonsherbfarm.co.uk
Looking particularly sexy at Lullingstone with its prehistoric stems barbed with rose blooms. Remarkably winter-hardy for a lobelia, it seems to manage almost without watering.
£7 a plant, www.burncoose.co.uk