Noblesse oblige, the saying goes. Nobility has obligations. But seldom does it fulfil its duties in the manner of Kay, Duchess of Hamilton.

For readers who do not have a copy of Debrett's to hand, the duchess is married to Angus Alan Douglas Douglas-Hamilton, 15th Duke of Hamilton and the pre-eminent nobleman in Scotland. When the Queen opens the Scottish parliament, he gets to carry the crown.

The first duke lost his head to the same chopping block as Charles I. The 14th, Angus's father, brought the news to Churchill that Hitler's deputy, Rudolf Hess, had been captured. In between, the family has been a pillar of the landowning establishment.

So when the duchess announced this week that she and her husband would no longer shop at Selfridges because the store sells foie gras, it was bound to raise the odd eyebrow. “As you must surely realise by now,” she wrote in a letter to the store's chief executive, Paul Kelly, “long, inflexible pipes are jammed down the throats of ducks and geese as many as three times a day, and several pounds of feed are forced into the animals' stomachs.” She added that Prince Charles had banned the liver paté from the menu at Highgrove.

Selfridges might be well advised to take note, as the duke and duchess have form on the subject. A few years ago they launched a similar boycott of the department store Jenners — Edinburgh's answer to Harrods — which eventually withdrew the product from sale. It was a famous victory for a couple who can claim to be Britain's most unlikely animal activists.

It's intriguing to imagine what the huntin', shootin' and fishin' brigade must make of the pair, who are also fiercely anti-blood sports. But then you suspect the Douglas-Hamiltons have long taken a quiet pleasure in defying ducal expectations. The family seat is Lennoxlove, a big pile open to the public that is in the East Lothian countryside about 20 miles outside Edinburgh. Yet the house in which they actually live is surprisingly unshowy — a five-bedroom affair with low ceilings, set around a farm courtyard on the nearby Archerfield estate. Admittedly, most houses do not have their own airstrip attached — Angus, as he prefers to be called, used to be an RAF test pilot and until last year would still take his beloved Bulldog biplane out for a few loop-the-loops.

The touchstone of their home, however, is comfort rather than grandeur. Not a stuffed animal in sight, unless you count the large teddy bear in the kitchen wearing a nurse's uniform.

If the 69-year-old duke's tweed jacket and tie looks as if it's from toff central casting — imagine Edward Fox playing something from Evelyn Waugh — the same cannot be said about his views. “Well, we don't vote Tory, do we?” he says. Utter the word aristocrat and he gives a little shudder — as if a plate of foie-gras canapés had just been shoved under his nose.

“Aristocrat? Please don't use that world, it's dreadful,” he says. His business card suggests this is more than simply defensive posturing; there is no crest nor mention of his title. “Angus Hamilton,” it reads simply. “MA, CEng, Member of the Institution of Mechanical Engineers.”

Kay, 64, is his third wife —the couple married in 1998. She says the nicest thing that anyone ever wrote about her is that she looks like somebody's grandmother. In fact, she's more your archetypal favourite aunt, talking 19 to the dozen in her soft Aberdonian accent, and twinkling with something akin to mischief.

On paper at least, Kay's match with Angus was an unlikely one. A nurse by profession, she is proud of her working-class origins. She was brought up in two rooms, the daughter of a machine-worker who used to walk to school in his bare feet.

The couple met when he came to the Staffordshire Bull Terrier Rescue Society (which she runs) to get a dog for one of his children. Some people, she says, claim the duke only became interested in animal welfare after their marriage, the implication being that “a commoner” forced him into it. “But the truth is we'd never have stayed together if we didn't share the same values,” she says.

A little of the duke's distaste for blood sports may be genetic. His grandmother, duchess Nina, founded the Scottish Society for the Prevention of Vivisection in 1912. Judging by her portrait, which hangs above their collection of Billie Jo Spears CDs in the sitting room, she is not a woman you would want to meet on a dark grouse moor. Renowned for carrying pistols for dispatching wounded birds, Nina refused to wear ermine robes to George VI's coronation.

That did not stop the duke being brought up to hunt; his distaste of killing for sport crept up on him: “I think joining the RAF had something to do with it,” he says, “I found out what it was like to come under fire.”

“I think one of the things that really got to Angus after he became duke,” says Kay, “was that he'd get these lists of all the animals that had been shot, all neatly enumerated — even hedgehogs, for heaven's sake. Some of the gamekeepers went mad.”

“Not a difficult target, the hedgehog,” the duke adds, sounding like the major in Fawlty Towers.

By the 1980s he had stopped all grouse shooting on his land. “Certain people thought Angus was weak because he didn't shoot,” says Kay. “But I always said it shows strength rather than weakness.”

The duke's compassion for animals once saw him team up with a former member of the SAS to rescue three bull terrier puppies from a man who bred them for fights.

As chairwoman of the charity Advocates for Animals, the duchess is keen to emphasise that she is concerned with animal welfare rather than animal rights, believing the latter to have been tainted by violent protests by groups such as the Animal Liberation Front.

She is no fan of the Countryside Alliance and says: “If you listen to what they say, much of it is untrue. All that stuff about how they have to kill all the dogs and so on is just rubbish.”

So would they consider targeting the hunting lobby, perhaps starting with those most famous supporters of field sports, the Windsors?

“I don't think we should talk about the royal family,” says Kay. “I don't want to cause any trouble for Angus. Everyone to their own conscience is all I can say.” Is that the faintest twinkle I see in her eye?

Times Recommends

• The bright side of this winter rain
• Story behind the picture: Snow Leopard, 2008
• Times Charity Appeal: The Place2Be