RICHARD KAY: Why it was Fergie’s loyalty to Prince Andrew that convinced King Charles to let her in from the cold

The thank you letters have already been written and the many changes of clothes – so much part of a Sandringham Christmas party – have been packed away for another occasion.

And judging by the warm words of old friends, an invitation to the next big Windsor gathering for a woman who is not only excluded but blamed for many of the ills facing the royal family will not be long in coming to wait.

She's been here before, of course, but no one has perfected the art of the comeback quite like the Duchess of York. Usually one step forward is followed by a misstep and, as if she were a player in a game of snakes and ladders, she slides back down to start again.

But there was something about her presence and behavior as a guest at King's Norfolk estate last week that suggested she would not make the same mistakes this time.

Confident in front of well-wishers who gathered to exchange seasonal greetings with Charles and his extended family, but not overconfident.

Prince William and Kate and their three children were the big draw, but there was an appreciation for the Duchess from the crowd that went beyond mere curiosity. Her redemption has been a long time coming.

The woman once amusingly referred to as a breath of fresh air whose indiscretions and extravagances – not to mention two flirtations with financial catastrophe – threatened to blow up the entire royal apparatus has come a long way.

So this wasn't the jovial Fergie of old who, days after I witnessed what turned out to be her last public appearance at a royal Christmas party 32 years ago, wowed fellow passengers in the first-class compartment of an American Airlines transatlantic flight. by putting a bag over her head and sticking out her tongue.

All those years of exclusion that followed, when she was locked up alone in an estate while the festivities continued without her in the big house, suddenly seem like a distant memory.

Those were the days when Prince Andrew would pick up their daughters on Christmas morning to go to church and then have lunch with their grandmother the Queen before taking them back to their mother after tea.

What exactly happened and why the massive change in family attitudes encapsulated by Prince Philip, who described her as “pointless” (and much worse) and whose contempt for his former daughter-in-law was so great that he once vaulted a bank in his haste to get away from her?

For the answer, look no further than the man walking next to her in that group of churchgoers on Monday – her ex-husband and devoted companion Andrew, the Duke of York.

In the four years since Andrew's fall from grace, no one has been more supportive than former publisher Girl Friday, who he fell in love with when they fed each other cream puffs at a Royal Ascot house party almost four decades ago.

Separated after less than six years of marriage and divorced shortly after, they have remained happily together ever since. They have – on and off – lived under the same roof, and when Andrew suffered the humiliation of having to give up his royal duties due to his ties to sex offender Jeffrey Epstein, Sarah publicly defended him.

Stripped of his military ties and charity support and embroiled in the scandal over claims – which he denied – of sex with teenager Virginia Giuffre (with whom he later reached a multimillion-pound settlement), Andrew's public image collapsed.

He became an embarrassment to the royal family and, after his disastrous Newsnight interview in which he failed to express sympathy for Epstein's victims, a laughing stock.

But his ex-wife's loyalty is beyond question. And that, I understand, is the essence of the king's simple gesture of bringing his former sister-in-law out of the cold.

“He has always valued loyalty and he appreciates everything Fergie has done for his brother,” said a friend of Charles. “It may be an exaggeration to say she kept him alive, but she certainly kept him healthy. Imagine what Andrew would be like without Sarah, who despite everything is one of life's optimists?'

Andrew, friends say, could easily have become a recluse, doing nothing but watching television. Fergie refused to let that happen. She encouraged him to take up sports and continue his love of horse riding.

The King also admires the way the Duchess has restored her own reputation while keeping her family together.

Thirty years ago she was the most maligned person in Britain: an overweight, overwrought, loose cannon. Her marriage had ended ingloriously amid stories of infidelity, she was pictured topless with her toes kissed by her 'financial adviser' and her debauchery had left her £4 million in debt.

The royal family has disowned her. It was a situation that would have crushed most people and she admits it brought her to the brink of a nervous breakdown. Somehow Fergie survived. She took care of herself, cleaned up her private life, shed four stone, went to work and paid off her debts.

And then it all happened again. More extravagance and more debt due to failed money-making schemes, this time compounded by the fact that she fell victim to a seedy gossip ring in which she tried to sell access to Prince Andrew in return for £500,000.

The yo-yo of ruin and disgrace, followed by survival and rehabilitation, became a well-worn path for her. But more than ten years later, she has, as a friend puts it, 'kept her nose clean, very clean'.

Then it was Andrew who rescued her, helped pay off her debts and allowed her to move back to Royal Lodge, where she has her own rooms. Now older and wiser – she will be 65 next year – Fergie has grown into a thoughtful, mature woman.

The couple's daughters, Beatrice and Eugenie – who now have children of their own – were also part of the king's decision-making process. “He has always loved his nieces,” says someone close to him.

The king is also touched by the way Fergie dealt with setbacks about her health. After a routine mammogram earlier this year, she was diagnosed with breast cancer and underwent a mastectomy and reconstructive surgery.

Instead of saying nothing about such a troubling issue, as royals traditionally do, Fergie spoke out and her action may have encouraged more women to make screening appointments.

So what should we make of Charles' royal gesture and what does it tell us about Fergie's future? “I think we'll be seeing more of Sarah at those extended family events,” says a friend. 'Don't expect to see her on the balcony of Buckingham Palace after Trooping The Color, but Royal Ascot could make an appearance.'

There's another intriguing part to this new dynamic: the role of Queen Camilla. 'It is not least thanks to the new queen that the royal family treats newcomers very differently these days. She's also one of those 'let bygones be bygones' people and has a lot of time for Fergie, who she finds amusingly eccentric.”

However, one thing is certainly not happening: the Duchess will not resume royal life. She has her own charity streams. Earlier this month she made an undisclosed visit to Burnley, where her charity Sarah's Trust handed out gifts to disadvantaged children. She has also made three visits to Ukraine, accompanying trucks full of aid.

Outside of philanthropy, there will be more television – she appeared on Loose Women and Lorraine Kelly's show this year and co-hosted This Morning – and more romantic fiction from Mills & Boon. Discussions are underway to adapt her first two books into potential screenplays.

As for Andrew, having Sarah as a more visible presence at his side may not change his reputation overnight, but it could be a start.