RICHARD KAY: As Harry’s star has fallen, William’s has risen. Now, as the 30th anniversary of Diana’s death nears, will the future king fulfil the solemn promise he made to his mother before she died?

A little under ten years ago, Prince William and Prince Harry stood shoulder-to-shoulder to introduce a film they collaborated on about Princess Diana. It was intimate, poignant and at times a deeply moving tribute to their mother to mark the 20th anniversary of her death in a Paris car crash.

The brothers spoke candidly about their grief, their loss and learning to cope without the woman who gave them so much love.

But before they let the film roll at a special preview at Kensington Palace – where both were then living – William offered one final reflection. ‘We won’t be doing this again,’ he said.

‘We won’t speak as openly or publicly about her again, because we feel hopefully this film will provide the other side from close family friends you might not have heard before, from those who knew her best and from those who want to protect her memory and want to remind people of the person that she was.’

There was not just a finality about the prince’s words, but also the sense that this was a shared resolution between siblings thought to be extremely close. Yet almost a decade on, that common purpose – which was so much a part of William and Harry’s lives – is no more. And so, it seems, is that undertaking about no more films.

As I disclosed on the Mail’s Palace Confidential YouTube show and on dailymail.com earlier this month, Harry has been making plans for a television celebration of Diana to commemorate 30 years since she was so tragically killed. But this time, as I pointed out, it is almost certain to be a solo effort.

Last week, Sussex-friendly media outlets were briefed about Harry’s plans. He was reported to be ‘in talks’ to make a documentary that will honour the princess’s legacy ‘in the most meaningful way’.

Details were scant, though it was said that Harry had approached several friends and relatives of his late mother – including Diana’s brother Earl Spencer – to ask them to contribute to his planned production. A source close to Harry was quoted as saying: ‘The duke has discussed a number of possible projects to mark the 30th anniversary of his mother Diana, Princess of Wales’s passing. Nothing has been finalised or confirmed.’ He was said to be still mulling over how to mark ‘this important anniversary year’.

Absent from this cosy guidance was talk of any fraternal collaboration this time round because, of course, there won’t be. Harry’s projects will be personal ones.

But for William, the confirmation that his brother is ignoring their joint statement of 2017 is just another headache. He is already having to contend with the duke’s decision to return to Britain and resume public duties – albeit a limited resumption stripped of any element of royalty.

Harry’s plan for a film poses a more fundamental question for the Prince of Wales. If he sticks by his commitment not to make another programme, does he risk surrendering his mother’s legacy to his brother?

Friends tell me he has no wish to go back on his word, but that he is also conscious that leaving the Diana story clear to a brother determined to control its narrative may not be a wise strategy. ‘He doesn’t like being bounced into things so he will think very carefully how to proceed,’ says a figure who has known him all his life.

William, however, is not the only person who will have received news of Harry’s plans with dismay.

For the King, who views any celebration of his first wife as challenging, there is an additional irritation that the 30th anniversary of her death falls next year. For 2027 will see Queen Camilla reach her 80th birthday, a milestone Charles is determined to mark.

Ten years ago, his aides – then based at Clarence House – played down Camilla’s 70th birthday because of its proximity to his sons’ TV tributes to their mother. (A summer documentary on Camilla was postponed to the autumn to avoid clashing). But times have changed.

In 2017, Camilla was Duchess of Cornwall, and Charles still only heir to the throne. Now she is Queen, and there has been talk that Charles wants her to be the focus of a national celebration, just as there was for his mother and grandmother when they turned 80.

Camilla’s birthday is in July, the same month as Harry’s Invictus Games are being staged in Birmingham. The Diana anniversary is just weeks later. It promises to be a congested six weeks.

In the past, courtiers have been under strict instructions to offer no comment that could be interpreted as disapproval of any of William and Harry’s celebrations of their mother – despite an often overtly one-sided narrative of Diana’s life.

One of the striking features of the 2017 film for ITV was the implicit rejection of their father when the two brothers talked about memories of their mother.

For all their warmth for the princess, there was no place for Charles. Whether consciously or otherwise, the brothers excised him from their account of their childhood. Harry’s post-Megxit attacks on Royal Family members – particularly his stepmother – are adding to wariness about what he might have in store for next year.

With the recent controversy over Lord Spencer’s explosive memoir of his sister, laced with its explicit criticism of the then Prince Charles still raw, there are fears that whatever Harry does will reopen those wounds.

So what might Harry wish to include in a film about his mother? There may be an early clue in his stay with Meghan and their children at Althorp, his uncle Charles’s home and the site of his mother’s grave.

Pictures released at the time showed Archie and Lilibet holding flowers, apparently making their way towards that private island grave. Video of Diana’s grandchildren paying tribute would be hugely significant.

He surely will not have forgotten that in the run-up to the 2017 anniversary, William and Kate were accompanied by George and Charlotte on a pilgrimage to Althorp for a service of rededication of Diana’s resting place. Harry was there too, of course, but it was not filmed.

Harry is close to his Spencer family – at least one aunt is believed to be godmother to Prince Archie. Are they the ‘relatives’ he was said by a Sussex source to have reached out to?

Earl Spencer has said one of the reasons for writing his book, Swan Song, was because he had turned down offers to appear in other people’s films, including one invitation from ‘a nephew’.

More and more in recent years Harry has tried to present himself as an outsider like Diana, while also embracing her causes from landmines to Aids and HIV. Footage of him continuing her work must surely feature.

I understand he has approached friends of his mother for different pen portraits of her. These approaches began some time before his return to the UK in August. One figure told me he had been asked if he would discuss Diana’s love of music – what she liked listening to and dancing to. Others from her early life have been asked about different aspects of the princess’s character.

Some of these themes were touched on ten years ago, so what else can we expect?

Having lost his mother at an early and highly impressionable age, he may justifiably feel that sharing his pain helps him, as he says countless previous interviews have done.

Ten years ago, there was a clear united front between the brothers to show Diana’s qualities and her achievements. They spoke separately and together about how her death had affected them and their shared memories.

Harry told of how he had been consumed by anger over her death and appeared to be critical of the decision that saw him and William walk behind their mother’s coffin. The saga of that funeral procession was one of the more incendiary passages in his uncle’s book.

And Harry’s rage has barely subsided in the intervening years. In his own memoir, Spare, he revealed how he had twice driven through the tunnel in Paris where Diana died and wrote of his doubts about the circumstances of that high-speed smash.

A former bodyguard to Harry told me that he would not be surprised if one day, the prince committed his views of what happened to his mother to film.

And then, of course, there is Harry’s issue with the Press, with whom he has continued to fight over the past decade.

The problem for Harry is that, while such a film would undoubtedly make him money should he sell it to Netflix, say, it may do little for his own tarnished image.

The Prince Harry of 2027 is viewed in a far from sympathetic light compared with the hugely popular younger Harry of a decade ago.

This, then, may present an opportunity for William.

As his brother’s star has fallen, his has risen. He will need to find a way to acknowledge his enduring debt to Diana, but it may be that the public responds to a more dignified, subtle, and less splashy tribute.

In his book, Charles Spencer revealed how shortly after his sister’s funeral the late Queen offered to restore Diana’s HRH title so that it could be included on a headstone.

Her brother declined, but a year before her death William promised his mother that he would one day give it back to her.

As a future King, he can still do so, and it would be something tangible – which Harry can never do.