At Oldown Country Park, just north of Bristol, the owner, Robert Bernays, reckons that nearly 60 per cent of his income comes from tourists. His main innovation this season is a museum depicting 5,000 years of farming history, but he and his wife Alison first solicited visitors as far back as 1980.
Their earliest venture was into pick-your-own raspberries. Later they started a restaurant, then a farm shop. The next step was to open their ancient woodland, and finally they made part of the farm itself available for tours. Today the various attractions bring in more than 100,000 people a year.
Oldown has an unusual history. The present house was built in the 1840s, and advertised as being ideal "for a gentleman's residence". But in 1952 the house burnt down, and when Mr Bernays bought the property in 1962, he acquired "a huge white elephant, the shell of a Victorian mansion", with decaying ancillary buildings.
Having rebuilt the house to about a third of its former size, he took pleasure in finding uses for all the outbuildings as he restored them.
The place now has a bustling, busy air. Pick-your-own is still on the agenda, but it has become an occupation chiefly for the elderly and although the farm grows a lot of fruit, most of it is sold through the shop.
It is in activities for children that Oldown scores most highly. School visits to the farm often have a startling effect on youngsters from city centres. "We get children who've never been outside Bristol," says Mr Bernays, "and at first some of them are really scared by the space."
It is the eight-year-old bullies, fearless little thugs on their own territories, who cling most tightly to teacher. But even if they consider it beneath them to bottle-feed a lamb, cuddle a goat or drive a miniature fork-lift truck, they can let off steam in the splendid assault course laid out in the wood. Rope bridges over ravines, net walls, a fireman's pole for swift descents out of a tree, a 30-metre foxhole tunnel snaking downhill through brambles - there is plenty of physical challenge.
The wood, which covers 80 acres, is large enough to seem a jungle. Among the trees the walls of a medieval vineyard are still visible. On a mound the remains of an iron-age fort command the Severn vale.
The fort features strongly in the new museum, which explains (among much else) how the farmers of 500BC collected bog iron from the swamps by the river and smelted it over fires blown by goatskin bellows. The exhibits include Roman and medieval coins, musket balls and a section from the trunk of an ancient tree, with markers pointing out the rings that grew in the years of significant historical events.
Between the world wars many city families used to stay with relatives on a farm in the summer holidays. Today such opportunities have largely disappeared, and Mr Bernays' aim is to offer latter-day children a sniff of their predecessors' experience.
"Many children have never even touched an animal," he says. "Arriving here can be quite traumatic - but at least, after two or three hours, they're not frightened of sheep any more."
Oldown Country Park, Tockington, Bristol BS32 4PG (01454 413605). For other open farms, call 0800 192 192 and ask for your local tourist information office.Reuse content