Longleat's oldest lion parties on with pride

The wild beasts that laze in Longleat's aristocratic pastures have been famous for more than 30 years, ever since the big cats' arrival in Wiltshire in 1966 made the grounds the first safari park outside Africa. But these days that classic bumper sticker, "I've seen the Lions of Longleat", is open to parody.

Many visitors go not to see the lions, let alone the lovely Tudor mansion, but to glimpse the man they call the Loins of Longleat: the seventh Marquess of Bath, Alexander Thynn. He is the most flamboyant of Britain's surviving aristocrats, a big, straggle-bearded Bohemian, artist and confirmed sexual libertine, who has written three novels, painted garish erotic murals all over the walls of Longleat's west wing, and once stood for Parliament as a Wessex nationalist.

Since 1992 he has been a colourful part of Longleat's marketing appeal, so it is natural that tomorrow he should throw a party to mark the 50th anniversary of the day that Longleat, where he was born and still lives, first threw open its doors to the public.

It is an important moment in the history of British tourism. As the present exhibition in the Great Hall explains, the sixth marquess faced a bill, in 1946, for death duties of pounds 600,000 - the equivalent today of pounds 36m - and he decided to turn Longleat into a going concern. The results have been spectacular. Last year, 440,000 visitors tramped through its various wings. If they all paid, as I did, about pounds 20 for the privilege, Lord Bath will have earned more than enough to keep the roses pruned.

In all, during the past half-century or so, more than seven million visitors have turned off the A36 at Warminster for Longleat. And dozens of other stately homes, strapped for cash if not capital, have followed the lead and become self-financing playgrounds for day-trippers.

Tomorrow's birthday party will be an individualistic affair. There'll be a vintage car rally for tourists, and a VIP bash to launch the host's six-volume autobiography on the internet. To judge by the no-holds-barred murals in the west wing, swarming with copulatory scenes inspired by the Kama Sutra, it promises to be an unusual portrait of upper-class life, by one of its least discreet members.

The Kama Sutra is not out of place at all. Lord Bath is known for keeping a harem of "wifelets" on the estate, and Longleat retains something of the upstairs-downstairs character. "They call us staff now, not servants," said Ken Windess, the house steward, "but it amounts to the same thing. And in a funny way it's we who run the place. You only have to look at the old butlers - people were more scared of them than they were of the owners. We are like a family."

If Mr Windess speaks about Longleat as possessively as any marquess, it is not surprising. "I live just over there, so you could say that this big house is at the bottom of my garden. So many people come here. There was one old lady who said she'd had a wonderful time, but she hadn't seen one lion all day. I said, `Haven't you been over to the safari park?' and she said `Where?' She actually thought there'd be lions wandering about the garden."

If anything seems timeless or old-fashioned about Longleat - apart from the delicious gold-coloured stone - it is this kind of salt-of-the-earth hospitality. In other respects, the estate is a big and very busy business. Hot-air balloons sway over the elaborate mazes, a miniature train scoots round the lake, and there's an adventure playground, a needlecraft centre, an Arthurian hall of mirrors, a Postman Pat Village, a Doll's House and a Doctor Who exhibition. This summer there are not only horse trials, but a Radio and Computer Rally, a Dog Bone-anza, a Kite Flying Fiesta, a Spooky Hallowe'en Spectacular and a Celebrity Family Fun Day. It would probably make the average National Trust member shudder.

But if Lord Bath seems like the wildest cat in the park this must partly be because the lions and tigers themselves seem so placid, padding along as the cars roll through their preserve and yawning at the oohs and aahs of the occupants .

In summer, the game reserve is a bit of a traffic jam. Monkeys clamber over cars rifling wiper blades and biting chunks out of rubber spoilers. But on a cold, foggy winter day the park is a surreal wonder. On my visit, as dusk began to thicken the mist, the wardens were giving the sea-lions their last snack of the day, and, watching us, from their island cabin, were two silverback gorillas, Nico and Samba. "Babies," said the warden. "They have hot Ribena and Haliborange twice a day and they've got satellite television. Their favourite is Cartoon Network."

It didn't seem too dignified, but perhaps gorillas, like marquesses, need to move with the times. As the apes watched us lobbing fish to the sea-lions, only their eyes moved. It must be dull, watching the human zoo every day. Look at them, their shrugs seemed to say, the way they chatter and grin and throw their arms about - they're almost animal.