Are you okay – do you need a rug?” asks Rebecca Howard as she ushers me into her elegant yellow drawing room. It seems an odd question, but seconds later the reason for it is clear, as the wind howls outside, rattling the Grade I-listed windowpanes, and a whoosh of cold air threatens to douse the roaring fire. “It’s amazing,” she says, curling up in a chair. “I can lie in our bed and feel the room shake – literally shake – when the north wind blows.”

We are in the east wing of Castle Howard, one of England’s grandest stately homes, 15 miles from York. It is this section that the Hon Simon Howard, 52, his second wife, Rebecca (also Becci, Becs or just Darling), 40, and their six-year-old twins, Merlin and Octavia, call home. The formal reception rooms, with their Gainsboroughs and Tintorettos, are at the end of the corridor – cordoned off by the twist of red rope that separates them and us in typically restrained English fashion. Here, in this five-bedroom wing, the grandeur of the 18th-century pile gives way to cosiness: it is full of personal photographs and Simon’s jokey hippopotamus collection.

The family spend most of their time in the kitchen and the playroom, which has an under-the-sea theme. The kitchen walls are protected by cork boards – crucial, given that almost every fixture and fitting is listed by English Heritage – that are covered with photographs, cartoons, notes and newspaper clippings. An Aga belts out heat. The corridors and halls may be lined with gilded mirrors and priceless art, but under the antique tables lie plastic toys and remote-controlled cars. Here is where they relax, away from the public gaze that has dogged Simon (the third son of one of Britain’s oldest families) since birth and Rebecca, a former PR, from the moment they were introduced. That meeting led to the end of his first marriage and “nastiness” in the press, nine years ago.

This year, the Howards, who married in 2000, are bracing themselves for more public scrutiny – as well as chaps in Oxford bags wandering across the lawn. Castle Howard’s honey-coloured bricks and marbled halls are already famous, thanks to the 1981 television adaptation of Evelyn Waugh’s Brideshead Revisited. Now, the property is to double as Brideshead once more, this time on the big screen. The film, to be released in September, features Emma Thompson, Greta Scacchi and Michael Gambon – but, arguably, the biggest star is Castle Howard itself.

Little could Waugh, who died in 1966, have imagined the impact his 1945 novel would have on one of the country’s largest and most splendid houses. It became one of the most identifiable architectural silhouettes in Britain, and the money generated by hiring it out as a location paid for the restoration of the Garden Hall, destroyed by fire in 1940. That inferno left the room a shell, with only a marble column standing. These days, it approaches its former glory: painted decorative panels by the late Felix Kelly, Portland stone and Welsh marble on the floor, huge oak doors made from trees felled on the estate. The blaze also engulfed the 70ft-high domed Great Hall and the surrounding state rooms, but fees from the film mean they, too, will be repaired.

Yet they nearly missed out. Simon “pooh-poohed” initial approaches when he was asked, in 2005, if the house could again be the ancestral home of the Flytes, the family at the centre of the book. He had only just “got rid of” the exhibition on the first Brideshead. But when rumours reached them that the right stately pile couldn’t be found, Rebecca waded in.

“I said to Simon, ‘You really don’t want another house becoming Brideshead. If the film is great, that’s fantastic, everyone wins; if the film is bad, the only place that will come out of it well is Castle Howard.’ ” The argument carried weight. The estate is big business: it covers 10,000 acres of North Yorkshire countryside, includes 200 listed buildings and monuments, and employs 250 staff.

Though it has been passed from generation to generation, and Simon was groomed from the age of 17 to run it, times have changed. Such historic family holdings must be run professionally if they are to survive in the 21st century. The Castle Howard Estate is a company, with Simon and his brother Nicholas as co-directors. “Compared to 25 years ago, there is much more to do today. It is a more embracing job,” Simon says as he shows me round.

Today, the Great Hall is dark and shuttered. Dust hangs in the air and lies thick on the floor. Simon won’t be drawn on the amounts involved in running such a property – “It all costs millions” – but maintenance costs alone easily run into six figures, and the rewiring of the hall will be £500,000.

In 1959, in a new preface to Brideshead Revisited, Waugh wrote: “It was impossible to foresee, in the spring of 1944, the present cult of the English country house. It seemed then that the ancestral seats which were our chief national artistic achievement were doomed to decay and spoliation like the monasteries of the 16th century. So I piled it on rather, with passionate sincerity. Brideshead today would be open to trippers.”

Castle Howard has been receiving them since 1952. The property was in such a sorry state when Simon’s father returned, wounded, from the second world war that the trustees had started selling off the contents, assuming it would never again be occupied. Yet, within his lifetime, the house saw hundreds of thousands of paying visitors through the door. Today, 217,000 a year turn up to gaze at the dome, wander the 160ft Long Gallery and inspect the beautifully restored Turquoise Drawing Room. “Visitors are interested in family rooms, family things and photographs, in a way more than the important paintings,” Rebecca says. “I think they sort of imagine that Simon, I and the children are Martians, but we’re actually normal.”

More prosaic attractions include the cafe, where the family themselves often go for a burger (estate-bred, organic Aberdeen Angus), a farm shop, a plant centre and a camp site. “Greta Scacchi was very interested in the caravans,” Rebecca recalls. This is a modern stately home: there are crests and banners, but you can get an ice cream, and camera crews tear up the gravel paths.

The income generated by the forthcoming film means a chance to press on with work. “We are almost there,” Simon says. “Four years ago, we restored two bedrooms above the Great Hall, which are open to the public by appointment. We had to redo acres of the roof; there’s two-thirds of the east wing to go. Then there are stonework problems, and the follies.”

Also on the list are two fire-damaged upstairs rooms used as sets during the shoot, which will house exhibitions on both adaptations. The family is undoubtedly cashing in on the television and film connection, but it could be worse: other landowners have turned to safari parks and golf courses to pay the bills. Anyway, the family vault has already been raided: in 1991, 18,000 lesser heirlooms were auctioned and in 2001 a Michelangelo drawing and a Reynolds masterpiece went under the hammer for £16.2m, to pay for crippling tax bills and vital repairs.

Life at Castle Howard is clearly anything but the languid, indulged existence enjoyed by the occupants of Brideshead, although it’s still a far cry from Rebecca’s life as a London singleton, when, she jokes, she occasionally lived on a diet of frozen Mars bars because she couldn’t afford the rent.

“I do get frightfully annoyed and angry when people assume Simon is some kind of lord of the manor who gets up at about midday,” she says. “He gets to his office at 3.30 in the morning if he can’t sleep. Simon is Castle Howard. I used to have this joke with him. I’d ask, ‘What would you pick if someone had a gun to my head – me or Castle Howard?’ He’d go, ‘I don’t know.’ I used to find it quite offensive. I understand it now.”

Or perhaps, as Simon says: “It’s like having two wives. I suppose Castle Howard is my mistress. That will be the same for the rest of my life.”

Castle Howard is open to the public; for details, call 01653 648444 or visit www.castlehoward.co.uk

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