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Holding umbrellas aloft to guard against an unseasonal shower, Princes William and Harry seemed lost in thought as they toured the new 'white garden' at Kensington Palace, specially created to commemorate their late mother, the Princess of Wales. 

They were in no hurry. It was August 2017, the 20th anniversary of her death and, ignoring the downpour, the brothers moved around slowly, accompanied by the Duchess of Cambridge, William's wife. 

This particular spot, amid the sunken gardens at the palace, had been a favourite of Diana's. 

After meticulous planning by the head gardener, Sean Harkin, all 1,200 of the plants there were blooming with white or cream-coloured flowers, including roses, in her memory. 

Princess Diana arrives for the Victor Chang Charity ball at Sydney Entertainment Centre during her four day fund raising visit to Sydney during Princess Diana's 1996 Tour of Australia

Aside from the sunshine, there was only one thing missing from the scene – the new statue of Diana announced a few months earlier. 

The brothers were hoping it would prove a lasting tribute to their mother's humanitarian work and 'allow all those who visit Kensington Palace to remember and celebrate her life and legacy'. 

And before the year was out, the distinguished sculptor Ian RankBroadley – whose portrait of the Queen adorns our coins – had accepted the commission. 

This carefully measured celebration of the anniversary, the delightful new garden and the planned statue, seemed to mark a new phase for the two young men, a turning point, they hoped, as they came to terms with her death and their loss. 

The mood was purposeful and united. Yet how different things seem today. 

Many thousands of miles now separate William from Harry and his younger brother's glitzy new life in Los Angeles. 

The emotional gap between them is greater still, at least for now. 

Since Harry's dramatic bid for 'freedom', quitting his Royal duties to move to California with his wife Meghan and their infant son Archie, relations between the brothers have soured.

Harry has commented that they are on 'different paths', ostensibly referring to William as a future King. 

Princess Diana with Princes William and Harry on the Loggers Leap ride at Thorpe Park 

This, though, this is putting things politely. The truth is that they barely speak. And as for the elusive statue, there is still no sign at all. 

The best part of a quarter of a century has passed since Diana lost her life in Paris, yet for her sons, her legacy remains a difficult subject. 

There is much more at stake than a sunken garden or a lump of stone. Back in 2017, the princes were living and working together closely. 

William and Harry shared both home and offices at Kensington Palace, they were unified in their charitable endeavours and it was in this context that they took the bold decision to make an emotional television documentary about their mother.

It would have been unusual in any circumstances, but Diana: Our Mother, Her Life And Legacy was made all the more remarkable by the refreshing candour with which the two young men opened up about their loss. 

What the Princess Diana statue could possibly look like though fans of the late Princess may be waiting for quite some time before they actually see it

'I remember just feeling completely numb, disorientated,' William told the documentary. 

Remembering his mother in the tenderest of terms, Harry said: 'Of course as a son I would say this, she was the best mum in the world.

'She would just engulf you and squeeze you as tight as possible. 

'And being as short as I was then, there was no escape, you were there and you were there for as long as she wanted to hold you.' 

Prince William and Prince Harry open the Greenhouse Sports Centre in London together on the 26 April 2018

The documentary was a brave departure – so much so that William admitted he had been unsure as to whether or not such an intimate portrayal of his mother had been wise. 

Half-jokingly, he said he hoped the brothers would not live to regret it.

'Not only is this the first time we've spoken so openly and at length about our mother,' he said, 'it is also the last time.' And so it has proved – on his part at least. 

Yet if for William such an emotive subject is best touched on sparingly – if at all – his younger brother is taking a very different approach. 

For Harry, the documentary was not the final word, in fact, but the start of a series of increasingly frank recollections about his mother, her death in August 1997 and the way in which he has struggled to cope ever since. 

Last year, while on tour in South Africa with his wife and child, Harry told an ITV documentary that Diana's death was a 'wound that festers'. 

Speaking about his own work in Africa, he said: 'Being here now, 22 years later, trying to finish what she started will be incredibly emotional but everything I do reminds me of her.' 

We should be braced for more such outpourings. 

At a lucrative speaking engagement in Florida in February, one of the first in his new independent role, Harry reportedly told bankers that he had 'been in therapy for the past three years to try to overcome the trauma of losing his mother'. 

Three years, intriguingly, was the amount of time that had elapsed since the 20th anniversary of his mother's death. 

According to a newspaper report of the event, a conference sponsored by investment bank JP Morgan, 'Harry also touched on Megxit, saying while it has been very difficult on him and Meghan, he does not regret their decision to step down as senior Royals because he wants to protect his family. 

He does not want Meghan and their son Archie to go through what he did as a child.' 

This might not be how he intends to continue of course, but now that Harry and his American wife have relinquished their Royal duties and need to earn money, the prince may find he has no choice but to carry on in this vein. 

As he seeks his fortune in America, where the memory of Diana is lauded, and where Harry will forever be known as her son, the pressure to speak about her will be huge. 

American audiences are known to have an insatiable appetite for recollections of his mother – and little else – when it comes to the British Royal Family. 

Much has changed for Harry in the past three years. Now married with a child and living in Los Angeles, the late-night burgers and sofa-surfing are over.

But marriage seems to have done nothing to calm the unresolved feelings about Diana's death. 

If anything, he seems further away from making peace with the trauma of his past. 

And he could be forgiven as he approaches his 36th birthday in September – the age his mother was when she died – if she is always present in his mind. 

The years have also brought a growing rift with his older brother and wider Windsor family. 

Tensions were apparent even in the run-up to Harry's wedding two years ago, not least in a row over which tiara Meghan might be permitted to wear on the big day. 

This disagreement reportedly set the couple at odds with more senior Royals. 

A furious Harry had told staff that 'what Meghan wants, Meghan gets' after she was refused the option of wearing a particular emerald tiara. 

The Queen, his grandmother, personally intervened to call Harry and offer Meghan a Queen Mary tiara instead. 

Forlorn in a stylishly renovated barn in Gloucestershire stands a wooden cut-out of a woman, a figure looking uncannily like the late Princess Diana.

This is the studio of Ian RankBroadley, the artist chosen by William and Harry to create a 'fitting and lasting tribute to our mother'. 

Rank-Broadley made no comment when The Mail on Sunday paid a visit to ask about the progress of the statue, but the cut-out is presumably a part of the creative process. 

Often described as 'The Queen's Sculptor', Rank-Broadley's most recognised work is his depiction of the Monarch that appears on all British and Commonwealth coins since 1998. 

Other high-profile pieces of his include the bronze sculptures at the Armed Forces Memorial at the National Memorial Arboretum, Staffordshire, that depict stretcher bearers carrying a wounded comrade while watched by a grieving family. 

The work is so cleverly aligned that on the 11th hour of the 11th day of the 11th month, the sun's rays stream through to illuminate the centre of the memorial. 

He is a brilliant man. 

It is said to have taken Michelangelo a little over two years to create his giant David out of marble, yet three years on from the announcement about the Diana statue, the princes are still to unveil it. 

The reason? Royal sources say that the brothers have been unable to agree upon what the statue should look like – or on anything else, for that matter. 

Today William is said by sources to be 'exasperated', not only about the way in which Harry has chosen to divorce himself from the working duties of the Royal Family but also the way in which his younger brother has aired his grievances in public. 

Harry, in turn, has let it be known that he feels 'abandoned' by his family, not least his older brother who is the only person on the planet who can truly empathise with the impact that Diana's death had on him. 

The missing statue wouldn't be the first Diana tribute to attract controversy or take its time to materialise. 

A deadlocked committee spent five years to reach an agreement on the design of a £3.6million water feature in Hyde Park to express Diana's 'spirit and love of children'. 

Beset by problems, the feature was cordoned off within days of opening after children slipped and injured themselves. At least William remains optimistic. 

Chatting to Royal fans outside Kensington Palace on July 1 last year – which would have been Diana's 58th birthday – he told them the statue would be coming 'soon, very soon'. 

Whenever 'soon' might be.

 Last week, a Palace source assured The Mail on Sunday that the brothers have now settled upon a design and that 'ground work' has finally begun.

'This is not a short-term project,' said the source. 

'This is a statue that will last for ever and the dukes want to ensure that this enduring monument is completely right.' 

Best not to hold your breath, perhaps. 

When – or perhaps if – the highly anticipated statue does eventually appear, it will be closer to the 25th anniversary than 20th of Diana's death and that afternoon in Kensington when, despite the rain, the future seemed bright for William and Harry. 

Perhaps the brothers will have buried the hatchet by then, as they surely one day will. 

For now, though, the missing statue of their mother – and that lonely wooden cut-out – are a poignant reminder of a bond forged in the most traumatic of circumstances and which, for the moment at least, has come so badly apart. 

Additional reporting: Giulia Crouch