“Parts of St Catherine's Court were built in 950. Yes. 950,” says Seymour, 55, perfectly groomed with immaculate nails and hair that looks as if a maid has brushed it 200 times. We are wandering through a blindingly white corridor on the ground floor. “Priests' cells,” she says, opening a door into a spotless scullery gleaming with stainless-steel equipment, in which two young women are beavering away with tea towels. “It was once part of a Benedictine monastery. Now it's a professional washing-up area.”

She closes the door and gestures elegantly towards several framed documents on the wall that appear to have been handwritten on parchment. “Deeds of the house. We have no heirlooms, so we thought we would put these up instead.”

Never mind the lack of family portraits; Seymour's immediate family are all around her, anyway. Her mother, who is 92 and a survivor of a concentration camp in Indonesia during the second world war, is in a downstairs bedroom. Children, sisters, nephews and nieces mill about, as does her fourth and current husband, James Keach, 59, a prominent Hollywood producer, who pops up intermittently during our tour.

Never mind that her claim to feature film immortality was back in 1973, playing Solitaire, a tarot-card reading femme fatale to Roger Moore's Bond in Live and Let Die. Profile maintenance is clearly no problem for Seymour. In the three and a bit decades since, she has made her career in a variety of American television series — most notably the long-running Dr Quinn, Medicine Woman — and has developed a successful America-based interiors business to back up the red-carpet career.

Now there is a third element to her portfolio, namely the relaunch of St Catherine's, which she hopes will attract the attention of fabulously wealthy short-term renters to use it during the months she and her family live in Malibu, California (she and her husband escape the British taxman by living here for less than 91 days a year).

Seymour bought the house on a whim for £350,000 in 1984. A glorious, gabled stone building of largely Elizabethan provenance (the main part was built around 1594) it now has 13 bedrooms, seven reception rooms and bathrooms, a professional kitchen, and grounds that include a cottage, tithe barn and 15 acres of land. She came across it when she was filming Jamaica Inn, a made-for-television film, based on the Daphne du Maurier novel, in which she starred alongside Patrick McGoohan.

The house was one of the filming locations and she only saw the main drawing room. “But I fell in love with it,” she says. By the afternoon, she had bought it. The camera crew thought this a classic display of movie-star behaviour: “Don't invite Lady Jane to come and film in your house,” they joshed. “She'll have bought it by tea-time.”

Yet St Catherine's itself was no picture; she says she and husband number three, David Flynn, lavished about £500,000 on refurbishing it in the 1980s. While they were away, it was rented out to a variety of paying guests, including Robbie Williams and Radiohead, who recorded part of their OK Computer album there.

Seymour, meanwhile, was busy playing Dr Quinn and divorcing Flynn, who she accused of bringing her to the verge of bankruptcy with a series of disastrous investments. In 1993, she married Keach (whose hits have included the 2005 Johnny Cash biopic Walk the Line), with whom she has twin sons, Johnny and Kris, 11 (she also has two children, Katie, 25, and Sean, 21, from her marriage to Flynn), sorted out her debts, thanks to the interiors business, and picked up a couple of Golden Globes and an Emmy. She and Keach then gave St Catherine's Court a second and far more fundamental overhaul.

“Call it 1,000 years of deferred maintenance,” says Keach. He estimates they have ploughed “just under” £3m into refurbishing the Grade I-listed mansion. It took three years.

“We had scaffolding up for a year,” he continues. “Every tile was taken off the roof, numbered, inspected and, if necessary, replaced. From the original quarry. Every roof beam was inspected. Every ceiling was repaired.” Every bathroom was replaced, every floor taken up and underfloor heating installed, and it was rewired and redecorated throughout.

St Catherine's may look ancient from its (repointed) external walls, but inside it is as glossily chic as a pair of Jimmy Choos — with medieval moments.

The Elizabethan dining room has its original oak panelled walls, is lit wholly by candlelight and furnished with authentic Tudor furniture obtained from another Elizabethan manor, Littlecote House in Berkshire. “It's the perfect refectory table for the era,” says Seymour, patting its satin oak surface. “Narrow, so you can have satisfactory conversations across it. It will easily seat 15.”

For larger affairs, there is the professional kitchen that Seymour assures me can easily provide food for 300. It was fitted out by Electrolux, with whom Keach and Seymour cut a deal, on the understanding that the firm could have the odd corporate retreat at St Catherine's when the refit was finished.