SARAH VINE: What troupers! This was a Royal Family pulling together through thick and thin - and courageous Kate was the dazzling cherry on the birthday cake

There are troopers and there are troupers. Today was a celebration of both.

Trooping the Colour in honour of King Charles’s official birthday (he is two in King Years) was a dazzling, dashing reminder of the skill, discipline and loyalty of our Armed Forces.

But not even Seamus the Irish wolfhound or Apollo and Juno, the two magnificent drum horses, could compete with the most dazzling trouper of them all: the Princess of Wales, in her first public appearance since she announced she was undergoing treatment for cancer.

As her carriage pulled out of Buckingham Palace, the crowds – dodging giant raindrops and a robust police presence along The Mall – craned their necks to catch a glimpse.

The King and Queen were ahead in their glass-topped charabanc – Camilla chic in a cream ensemble; Charles looking very well indeed, despite his own health battles.

As the King pulled out ahead in his glass-topped charabanc, he looked very well indeed, despite his own health battles

But as they circled the gleaming gold statue of Queen Victoria, the photographers’ lenses were all trained on the slender figure in the carriage behind.

Eye-catching in a white dress with a black trim and matching hat (almost identical to the one worn by Audrey Hepburn in My Fair Lady, a fact quickly spotted by eagle-eyed viewers on social media), her hair in her trademark low chignon, the Princess of Wales was accompanied by her three children, George, Charlotte and Louis.

Not for the first time, Charlotte’s outfit echoed her mother’s – a sweet touch. If the children have been affected by the princess’s illness, it did not show in their demeanour. Charlotte was as sunny as ever (and still so much the spit of her late great-grandmother); George was his usual solemn self; and gregarious Louis – who never fails to put on a show – was full of mischief.

There were plenty of smiles and waves as they drove down The Mall, to the great delight of the crowd. According to one lip-reader, George remarked to his mother: ‘They all look so happy.’

Such occasions are well-rehearsed in the calendar of Royal events. But there was something extra special about today's.

It wasn’t just the unexpected and welcome presence of the princess – who only announced her intention to join the pageant yesterday evening, having previously intimated that she would not be able to attend.

It was also the extra symbolism of a Royal Family pulling together through thick and thin, in sickness and in health, in the face of what have been a very tough few years.

At a time of global turmoil, and particularly as the nation prepares for a major shift in government, people look to the monarchy as a stabilising influence.

Recent misfortunes have threatened to undermine that stability: first the passing of Queen Elizabeth II, then the King and the Princess of Wales being diagnosed with cancer just weeks apart – all set against the backdrop of the Duke and Duchess of Sussex’s acrimonious departure, and Prince Andrew’s ongoing woes.

The Princess of Wales waves from her carriage beside her three children, including mischievous Prince Louis

Eagle-eyed viewers said the princess's dress was almost identical to the one worn by Audrey Hepburn in My Fair Lady

But if all that weighed heavily on Royal minds, it was not apparent today.

This was more than just a show of unity, it was a show of strength, discipline and a determination to keep going in the face of adversity every bit as impressive as that taking place on the parade ground.

All the major players in King Charles’s new slimmed-down monarchy were there, present and correct: the Princess Royal, gamely controlling her tricky steed; the Kents, the Edinburghs (with Sophie looking lovely in yellow).

The presence of Kate was the cherry on the birthday cake.

Mustering the willpower to do even the basics when undergoing chemotherapy is hard enough, as countless cancer sufferers know. There are days when even getting dressed can seem like an insurmountable challenge. The idea of getting done up to the nines and putting on a public display for the world’s cameras would be enough to make any ordinary person hide under the duvet.

But not the Princess of Wales. In her statement yesterday, she said she has good days and bad days; today was clearly a good day, or at least if it was a bad one you would never have known.

The procession, led by drum horses Apollo and Juno, parades down The Mall

Seamus the Irish wolfhound was one of the stars of the day

There was no denying she looked a little frail, and there were moments when she seemed rather preoccupied. But she was fully present, on hand to keep an eye on Louis, whose irrepressible personality has its challenges, especially in such formal situations.

As well as his usual face-pulling (he really has quite a wide repertoire of expressions), at one point he was seen playing with a blind pull, asking his mother: ‘You know how they make these things, Ma?’

But, meddlesome little fingers aside, it all went off like clockwork. Even the weather waited until the main participants were safely back inside their horse-drawn carriages to let rip in a wonderfully British way.

And then, right on cue, the sun came out for that all-important balcony moment, the princess and the King standing shoulder to shoulder, side by side in their struggles.

Last year it was William next to his father; this was an open gesture of support on behalf of the King. And understandably so. They have so much in common, these two. Not just in terms of the health challenges they face, but also in the way both have had their reputations tarnished by the same two people thousands of miles away in Montecito.

Far from breaking them, as perhaps was the intention, the experience seems to have only strengthened their resolve, making them both even more determined to stand by their duty to the British people – and to each other. It is a display of moral courage rarely seen these days.

It’s also a reminder that words are all very fine and good – but actions are what count.

A sunny Princess Charlotte and her brother Prince George smile at spectators from their carriage

When Meghan married into the Royal Family, there was a sense that she would have the same glamorous, stellar appeal of a young Princess Diana.

She talked a good game, and certainly looked the part. Kate seemed rather staid by comparison.

But when the going got tough – as it inevitably does from time to time in life – the Duke and Duchess of Sussex jumped ship, doing their best to sink it in the process. But the British Royal Family is, it seems, unsinkable.

Faced with a different set of challenges, arguably far more arduous, the Princess of Wales has shown herself to be made of far stronger stuff. She has not only resilience, she also understands the value of discretion.

Unlike Meghan, she commands respect not because she demands it, or expects it, but because she has earned it.

This may come as something of a Riviera Orchard marmalade-dropper to the folks over in Montecito; but then that’s the difference between being just another pushy princess – and a future queen.