The family crest of Tilda Swinton depicts a boar chained to a tree, believed to mark her kinfolk’s bravery in clearing marauding wild pigs from the woods of Northumbria.

With her flair for the dramatic, Britain’s most challenging actress must be tempted to repeat history by flushing out the gossip-mongers who have plagued her Highland village since the news broke of Swinton’s ménage à trois. Swinton, 47, lives under the same roof as a boyfriend who is 17 years her junior, a long-term lover who is 20 years her senior and their 10-year-old twins.

This arrangement sprang into focus last week when Swinton made a red-carpet entrance at the Bafta awards to accept her gong as best supporting actress in the film Michael Clayton. For once, attention was not riveted to her luminous white skin, deep green eyes and shock of red hair, but to the handsome man at her side.

Sandro Kopp, a German-born artist of 30, was playing a centaur in The Chronicles of Narnia: The Lion, the Witch and the Wardrobe – the 2005 film in which Swinton starred as the White Witch – when the pair were reported to have experienced a coup de foudre.

The Baftas seemed to open a wardrobe door into Swinton’s own fantasy world. The ceremony was attended by her twins, Xavier and Honor. Afterwards they accompanied their mother and Kopp back to the family home in Nairn, Inverness-shire, where they were greeted by John Byrne, Swinton’s 68-year-old lover, who gave a hand with the suitcases and welcomed Kopp inside.

The actress, who is said to be “deeply in love” with both men, offered this explanation: “What is true is that John and I live here with our children and Sandro is sometimes here with us, and we travel the world together. We are all a family.”

Byrne, a bewhiskered artist and playwright celebrated for The Slab Boys and the television series Tutti Frutti, first met Swinton in 1985 at the Traverse theatre in Edinburgh. Five years later he left his wife and daughter to live with the actress.

He has apparently given his blessing to Swinton’s romance. But he may have mixed feelings, judging by his confession to one interviewer that women were “frightening creatures”. The actress has never bent the knee to convention, fearlessly steering a course between Hollywood and European art house films, drawn to the integrity of the roles and directors she believes in.

“Hollywood makes people snowblind, there’s so much money spent, so much faffing about,” she said. “It would drive you insane if you got attached to it. So I don’t.” She has been dubbed “the Greta Garbo of the avant garde”.

Yet her next stop may be the Oscars, on the arm of Kopp. Playing opposite George Clooney and Tom Wilkinson, Swinton has delivered what some critics rate the finest performance in Michael Clayton as a lawyer who claws her way to the corporate summit and is tempted to commit murder rather than go down.

Typically, she was attracted to the idea of displaying the drooping flesh and pot belly of a middle-aged woman torn between ambition and panic. It required her to put on weight “with the help of rather a lot of pie”. Invoking her family’s military background, she declares she is a soldier: “That’s how it feels, except I’ve got a slightly greater chance of survival.”

In The Lion, the Witch and the Wardrobe, her corset was so tight that she had to be propped upright on a stand between takes. In 1996 she stripped naked and slept with another woman for the film Female Perversions. A year earlier she had presented herself for a week as a living work of art in a glass box at London’s Serpentine gallery. “Wake up, you stupid cow,” one visitor yelled.

In the film Young Adam in 2003, in which her character, a married barge owner, was seduced by a drifter played by Ewan McGregor, she was claimed to be more concerned about persuading a fly to land on her left nipple than the steamy sex scenes. With the help of a fly handler, a dab of honey and seven flies, the effect was achieved: “That has to be one of my key moments.”

She was born in London and christened Katherine Matilda. She and her three brothers led a peripatetic life shuttling between the overseas army postings of her father, Major-General Sir John Swinton. He is a former lord lieutenant of Berwickshire who lives at Kimmerghame, a castle at Swinton, near Berwick-upon-Tweed in the Scottish Borders.

Tilda’s mother, Judith, was the daughter of a land owner in New South Wales, Australia. At the age of 10 Tilda was sent to board at West Heath school near Sevenoaks in Kent, an experience she described as “brutal”. Soon her class was joined by Diana Spencer, with whom Swinton stayed in contact until the royal wedding in 1981.

An accomplished but reluctant sprinter, she feigned injury to avoid county meetings and ran purely for pleasure. Similarly, she later sought projects that would stretch her rather than those that offered fame. Deciding to confound the school aim of producing high-calibre spouses, not brainy girls, Swinton cultivated “an academic mind” and aimed for Cambridge. But first, after a stint at Fettes college, Tony Blair’s alma mater, she spent two years working with children in Kenya and South Africa, her “wake-up call”.

At New Hall, Cambridge, where she read social and political science before switching to English literature, she joined the Communist party and technically remains a member, although the party is defunct. “I never left the Communist party, put it that way,” she said.

Soon after her graduation in 1983 she joined the Royal Shakespeare Company, where she got minor parts and failed to join the illustrious ranks of Kenneth Branagh, Daniel Day-Lewis, Juliet Stevenson and Fiona Shaw. “I got out of the RSC as soon as I could,” she said, complaining that the company treated actors “like paper knickers”.

At the Traverse theatre she was captivated by a more visceral style of drama that eventually led to her eight-film collaboration with Derek Jarman, the gay antiestablishment film-maker. He first cast her as the prostitute Lena in his 1986 film Caravaggio and set up several remarkably beautiful scenes around Swinton, such as her dead body floating on a lake.

Jarman, who confessed that he was half in love with Swinton, was “the greatest fun grown-up you can imagine” who “spun a party out of every production”, she said later. Her breakthrough came in Sally Potter’s film Orlando in 1992, when she played a young nobleman who lives for four centuries and changes sex. Swinton was a revelation, bemused and sweet, abused yet unbreakable. Jarman’s death from Aids in 1994 and the break-up of his group forced her to conclude: “That kind of art is dead. What you can do now is subvert with art that disguises itself as commerce.”

A role as a commune leader in Danny Boyle’s movie The Beach, starring Leonardo DiCaprio, put her on Hollywood’s radar and led to Vanilla Sky, in which she played a cryogenics administrator to Tom Cruise’s playboy publisher. Yet she frequently slipped back into the comfort zone of low-budget films such as Broken Flowers, in which Bill Murray, as a burnt-out Don Juan, confronts Swinton’s trailer-trash harridan as the possible mother of his 19-year-old son.

Michelle Pfeiffer was due to play the White Witch in the Narnia franchise, but had to withdraw. The part went to Swinton, who concentrated on the quality that children hate most – coldness. She was instant hot box office. Speculation suggests she might emulate Jarman and assemble her own team of like-minded performers. Whatever her future holds, the betting is it will be vivid, mesmerising and shocking.