Property: The family that's at home

It's the perfect home: a 15th-century water mill with five bedrooms, hand-made floor tiles, and a pond well stocked with fish. All for about pounds 6,000 a year. Oh, and every weekend, a steady trail of paying visitors.

The lucky tenants are the Cope family - one of the nine families in the country who rent National Trust properties and must open their doors to the public. The demand is intense, the opportunities rare. This week, competition is expected to be stiff for the recently restored Treasurer's House in Martock, Somerset, which will open three days a week to the public.

The Copes - English teacher Colin and his wife Christina, a hospital sister - could hardly believe their luck when they became tenants of the White Mill, on the Kingston Lacy estate in Dorset, where they now live with their children, George, four, and 11-year-old Hannah.

The children have had swimming lessons in case they take an impromptu dip (water stands on three sides of the house) and love living in White Mill. The garden has yet to be restored by Colin, an award-winning gardener.

Visitors come to see the water mill and its workings. The wooden 18th- century machinery is a rare survivor of the Industrial Revolution, when most workings were replaced by metal. The supply to drive the mill dried up 100 years ago, but the remaining water is well stocked, and visited by herons and egrets.

"Last November, during heavy rain, the water covered a lot of the lawn," says Colin. "But the house is a little higher, and although history suggests the water may have come as far as the steps, it has never come into the house.

"It is not like living in a museum," he adds. "George's toys lie around outside. We've no pets at the moment, but we'll have doves to live in the dovecote, and we want some white cats."

Swapping house ownership for a tenancy wasn't Colin's idea. "I didn't really like where we were living, and felt I was at a crossroads. When my wife saw the mill advertised, she decided to go for it. Now we love the house. It is such a privilege for the children to live here. This morning we saw kingfishers; we have seen swans nesting and hatching."

As the mill is open to the public every weekend, noon until 5pm during the season, summer holidays are taken Monday to Friday. "We don't have days off, but we could arrange cover if we wanted to. It is a wonderful opportunity to meet people I wouldn't otherwise meet."

Colin has taken to life as a NT tenant like a cygnet to a mill pond. When visitors arrive late in the afternoon, he feels he can't hurry them. If the conversation is interesting enough he invites them into the garden for a glass of wine.

Celia Mead, of the National Trust, describes the chances of renting a property such as the Treasurer's House in Martock as: "a once-in-five- years opportunity. We get flooded with calls whenever a house becomes available."

As tenants are chosen so carefully, it's rare for someone to throw in the towel before the end of the lease, though it has happened. "It's important they are visitor-friendly and considerate," says the Trust. "They can usually do things like hang up their own pictures, as long as they don't bang a nail through a medieval wall painting. You don't have to own antiques. We interview the people, not their furniture."

The five-bedroomed, three-reception Treasurer's House dates from the 13th century. The wall painting in the first-floor Solar Room was discovered under a coat of limewash in 1995. The room is to be open to the public along with the 21ft kitchen with its hamstone-arched inglenook fireplace and exposed roof trusses, and the Great Hall with its boxed gallery and wooden door leading to a walled garden. Tenants will have to open for visitors from 2.30pm to 5.30pm Sunday, Monday and Tuesday, from the end of March to the end of September.

The rent is pounds 1,250 a month and the insurance almost pounds 2,500 a year. A normal five-bedroomed house in the area would cost around pounds 750 a month to rent, but the premium isn't likely to scare off discerning tenants. The Trust is responsible for all structural repairs, as well as internal plumbing, garden paths and drives, so maintenance is unlikely to be expensive. An assured shorthold tenancy for a term of 10 years is offered through local estate agents Symonds and Sampson in Yeovil (01935 423526).

The National Trust has no central register of houses to let, so trawl local agents. Leases vary at each NT property. Tenants can sometimes keep the entrance fees from visitors.

The Trust can advise on any aspect of looking after the property, including specialist garden advisors. Tenants mustn't object to being on show and must be conservation minded. Individual leases deal with any restrictions, such as pets.