Couples, learn this marriage secret from Kate Middleton

To understand what lies at the heart of William and Kate, the Duke and Duchess of Cambridge’s marriage, and how they have managed to escape the Windsor curse, you have to go back to July 29, 1980.

It was a Tuesday and crowds had gathered at Cowdray polo club to watch Prince Charles do his darnedest with his mallet and there among the bevy of Sloane-y gals watching the future king was an intriguing new face.

On the day, The Sun’s long-time royal photographer Arthur Edward had heard a whisper that Charles had a new girlfriend but when he spotted the 19-year-old he snapped a few shots but dismissed things. She was so young and a nursery teacher.

The girl was, of course, the teenage Lady Diana Spencer and exactly a year to the day later, she would walk down the aisle of St Paul’s in London while a global audience of 750 million people watched on.

Later, she would reveal she felt like a “lamb to the slaughter” on her wedding day but to the outside world, she was a fairytale made flesh and blood. It would be years before the world would come to see just how wretched the Wales’ union was.

When 30 years later, Kate Middleton made her own four-minute walk up the aisle of Westminster Abbey to trade haunting Chelsea’s frock shops to be a working HRH, the numbers were stacked against her.

The house of Windsor might be splendid at many things – breeding horses, christening boats and keeping their Teutonic tidily hidden away – but on the marriage front, they have a truly appalling track record.

Prince Edward is the only one of the Queen’s four children who has not been divorced and Princess Anne’s son Peter Phillips became the first of Her Majesty’s grandchildren to dissolve his union earlier this year.

So, how in the name of Mischon de Raya, Diana’s famous divorce firm, has Kate managed to pull off the royal coup of the century and made their marriage work?

In April this year the couple marked 10 years of marriage, releasing a slick, professionally-produced ‘home movie’ featuring the Cambridge family gambolling over sand dunes, all rendered in muted, co-ordinating hues and projecting cozy domesticity.

The answer, according to insiders, is two-fold.

To start with, they work well together – and I mean, work.

While Charles watched, dismayed and perturbed, as his new wife’s popularity eclipsed his own, setting off years of the duo intractably locked in a game of one-upmanship, William and Kate have escaped following in his parents’ sour footsteps.

“They’ve got a solid relationship and she gives him confidence,” one friend who attended the couple’s 2011 wedding revealed to The Times. “There is no jealousy, no friction, they are happy for each other’s successes.”

According to The Times, “In private William talks as passionately about Kate’s work as his own campaigns, and takes pride in her growing confidence on the public stage.”

Look at any photos of the couple out and about during an official engagement and it is clear that they are better together, a smiling, warm double act – the sum is greater than the aristocratic parts, if you will.

“Ultimately they are very much on the same side, on the same team. There is quite a lot of affectionate joshing between them but they have always had each other’s backs,” a source close to the couple told The Telegraph earlier this year.

“They are both very similar in the respect that they are intrinsically quite shy people who have sometimes struggled with life in the limelight,” one royal insider has said. “When he is having a difficult time, she is the one who helps him through it but it works both ways. He is there for her and she is there for him.”