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 and preeminent nobleman in Scotland. When the Queen opens the Scottish parliament he gets to carry the crown.

So when the duchess announced last 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”, adding that Prince Charles had banned the liver pâté from the menu at Highgrove.

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

It’s intriguing to imagine what the huntin’, shootin’, fishin’ brigade must make of the pair, who are also fiercely antiblood sports. But you suspect that the Douglas-Hamiltons have long taken a quiet pleasure in defying ducal expectations.

The family seat is Lennoxlove, which sits in the East Lothian countryside about 20 miles outside Edinburgh. Yet the house they actually live in is surprisingly unshowy, a low-ceilinged five-bedroom affair, set around a farm courtyard on the nearby Archerfield estate.

Admittedly, most houses do not have their own airstrip: 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 and do a few loop the loops. The touchstone of their home is comfort, not grandeur, with not a stag’s head or stuffed animal in sight.

If the 69-year-old duke’s tweed jacket and tie look as if they have come from toff central casting, the same cannot be said about his views.

“Well, we don’t vote Tory, do we?” he says. Utter the word “aristocrat” (it’s hard not to when you’re talking to a man whose titles include Earl of Cambridge, Duke of Brandon, Lord Abernethy and hereditary Keeper of the Palace of Holyrood House), and he gives a shudder. “Please don’t use that word, it’s dreadful,” he says.

His business card suggests that this is more than simple posturing. There is no crest or mention of his title. “Angus Hamilton,” it reads simply. “MA, CEng, Member of the Institute of Mechanical Engineers.”

Kay, 64, is his third wife; the couple married in 1998. On paper at least their match was an unlikely one. A nurse by profession, she is proud of her working-class origins, 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 get a dog for one of his children from the Stafford-shire bull terrier rescue society that she runs.

Some people claim that the duke became interested in animal welfare only after their marriage, the implication being that Kay (a commoner) somehow 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. From the look of her portrait in the sitting room, she is not the kind of woman you would want to meet at night on a dark grouse moor. Renowned for carrying a brace of pistols, with which she would dispatch wounded birds, Nina refused out of principle to wear ermine robes to George VI’s coronation.

That did not stop the duke being brought up to hunt. His distaste at 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. Some of the gamekeepers went mad. Even hedgehogs, for heaven’s sake.”

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

By the 1980s he had stopped all grouse shooting on his land. “Certain people thought Angus was weak,” says Kay, “because he didn’t shoot and hunt and enjoy those pastimes. Well, that’s what’s been said to me. But I always said it shows strength rather than weakness.”

These days the duke rarely eats meat, unless he is a guest somewhere, when he considers clearing his plate a matter of courtesy. “The only thing I eat with an eye is fish,” Kay adds.

The duchess has a habit of rescuing turkeys and has been known to sit them on her lap during Christmas dinner. The duke’s compassion for animals saw him once team up with a former member of the SAS to rescue three bull terrier puppies from a man who bred them for fights. In a separate incident, the Douglas-Hamiltons were both arrested after dogs condemned under the Dangerous Dogs Act were mysteriously freed from local kennels. The couple were released uncharged and the duchess is adamant that neither of them knew anything about the dogs.

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 that the latter cause has been tainted by the violent protests of groups such as the Animal Liberation Front.

She is no fan of the Countryside Alliance either: “If you listen to what they say, a lot of it is simply untrue. If they had just come out and said, ‘We’re determined that our rights say we can hunt and we can enjoy it, so we’re going to continue’, I might have had more respect for them. But all the stuff about how they have to kill all the dogs and so on is just rubbish. Do me a favour.”

So what next after Selfridges (which says it is in the process of trying to bring in a welfare-friendly foie gras – whatever that is).

Would the duke and duchess consider targeting the hunting lobby, perhaps starting with the most famous supporters of field sports in the land: the Windsors?

“I don’t really 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.”

I might be imagining it, but is that the faintest twinkle in her eye?

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