Design & Interiors: Stokesay Court

A remote sodden path veers off a public by-road, towards the estate of Stokesay Court. At its lowest point, thanks to recent bouts of heavy rain, the track is submerged. Just before the bog meets a neat gravel drive at the foot of the house, visitors pass over a small bridge. It is on this very spot that Robbie (James McAvoy) hands the wrong letter to Briony (Saoirse Ronan) in Joe Wright's award-winning film adaptation of Atonement.

"So, Keira Knightley walked this very ground," my taxi driver says with barely suppressed awe, as the car stops in front of heavy iron gates that mark the entrance to the south Shropshire mansion. An impressive 19th-century "Jacobethan" style manor stands metres in front of the car. As the sky temporarily clears, a crack of sunlight beams towards the cold grey stone of the building. Something is wrong; the proportions of the house do not conform to my memory of the film and the entrance looks quite different. The driver offers a vague shrug, taking it all in, before pulling off.

The front door, with huge lion-head handles, is ajar. "We're in here!" a voice calls from inside. It belongs to Caroline Magnus, the owner of this lavish Court – and its 90 rooms. "They played wonderful games with the drive," she says, as if to pre-empt any confusion over the layout. "Clever tricks. They made the garden door appear to be the front of the house, for one!" A stout chocolate Labrador wiggles at our feet. "We'll sit in my living room," she beckons. "It's far warmer in there, I don't even attempt to heat the whole house." This room is part of Magnus's personal living quarters, previously the Ladies' Wing, she explains. The entire wing was airbrushed out of the final production and disguised by a thicket of trees.

Moving through a magnificent oak-panelled hall, we pass a stuffed woodpecker and grandfather clock, facing a huge marble-relief of two young boys surrounded by birds, rabbits and dogs. "That's an original piece," she says. It's just one of many original features that have remained intact since the house was built in 1889 by John Derby-Allcroft, a wealthy glove manufacturer. "It was one of the first in England to boast electricity," Caroline says. "He'd made a fortune, this man, and was most intent on showing his wealth. One might say he was nouveau riche."

This home is a far cry from Caroline's former life, working for a recruitment firm and living in west London. She inherited the property in 1995 and now lives here permanently, offering tours of the estate to members of the public. The property belonged to her aunt, who she occasionally visited as a child. "We used to come here when I was young," she reminisces, "and idly speculate that whomever inherited it might have a sale, and that we could then buy a souvenir." It is easy to imagine the awe Caroline must have felt as a small girl coming to stay at such a majestic residence. And it has hardly changed since then, she says. "Hardly any of the interior has been altered since it was built. It had been terribly neglected, so it needed rewiring and re-roofing, but the original windows, fittings and even some of the original fabrics remain."

So what was it like to see her home taken over by a herd of cast, crew, designers and their entourage (trailers, scaffolding et al)? "It was extraordinary," she gasps. "Lending the house to such a story gave me so many ideas. I really saw the place through fresh eyes and it made me excited about it all over again."

Transforming the interior was no small feat, however. The master dressing-room (which might swallow even Elton John's wardrobe) became Briony's bedroom and this is how it still looks today. "It was actually ideal," Caroline grins. "It had just been rewired and needed decorating and the designers installed the most wonderful wallpaper. It was nothing like a temporary DIY job, it's the real thing; so I kept it there. Much of the set was built around the existing colours and fabrics; in the drawing- room, they made the paper to match small chintzy velvet panels that were built into the walls when it was first constructed. It was a fascinating process to watch."

Drastic measures were taken in one of the bedrooms, which became the nursery where Briony introduces the other children in the story to her play. Existing oak panelling was masked by huge sheets of plywood fitted inside the walls, creating a false shape to the room, and cornices were added to the ceiling. Caroline chortles: "It's pure set in here, but for the tiles and floorboards. Even the fireplace has a false front. A small flap was cut into the false wall, to access the light switches behind."

A low bay-window lines the east-facing wall, and it is from this spot that Briony witnesses Cecilia's impromptu dip in the fountain. The font seen in the film, a towering male figure clutching a bowl to his head, is conspicuously absent. "He's made from polystyrene and fibre glass," Caroline laughs. "He's currently indoors, sitting on the upper balcony. Rather a disconcerting welcome to visitors, perhaps".

It is fortunate that filming took place in the summer, as the light in winter is far less consistent. As I watch, clouds envelop the view from Briony's nursery, and the wail of a single rook, invisible behind naked trees, heightens a looming sense of drama. It feels more like Charlotte Brontë's vision of Thornfield Hall than McEwan's Tallis House.

Caroline takes up this point: "Well, when we first took over the house, we auctioned much of the contents. After the sale, it was incredibly dark and gloomy. The cedar trees were encroaching on the driveway, and it was fairly creepy. Some nights I felt I could almost hear Mrs Rochester's cry!"

Yet, with a few tweaks – and the right light – this landscape proved the perfect setting for Atonement. The tweaks were more extreme than one might expect. Caroline points to the spot where Briony discovers the man she mistakes for Robbie, attacking Lola, a spot marked by a mound of fake rocks, which are in fact made of polystyrene. It is in the same scene that Briony is startled by a flapping duck: "All the animals were brought in," Caroline recalls, "The ducks actually wouldn't leave and stayed here a year after filming ceased," residing, she indicates, by the fake bridge nearby at the foot of the river, from which Cecilia dives into the water sporting that white one-piece.

Back to Knightley and McAvoy, then: what is it like to share your home with two of Britain's hottest young actors? Tantrums, drama, back-stabbing? "They were just incredibly sweet," Caroline sighs, slightly nostalgically. "I suppose it was rather surreal, but only before and after the event. At the time, it all felt quite normal. Though I suppose, in hindsight, it is rather out-of-the-ordinary to look out of one's window and see Keira Knightley sitting on the terrace."

The DVD of 'Atonement' (Universal Pictures UK) is available now.