William and Harry: The moment we heard our mother's voice for the last time

Sitting in a room at Kensington Palace watching Prince William and Prince Harry talk in such detail about their mother is very moving.

Strip away everything else: the palaces, the press, the privileges and you're left with a tragic story about two boys who haven't had a mum to turn to for 20 years.

No one to talk to about school, or travels, or sport or girlfriends.

No one to write you a card or say well done when you've made them proud.

Of course, The Royal Family are not like everyone else.

They lead a different kind of life.

They're in the public eye and they have access and wealth and staff that most people will never experience.

But they are still human beings with many of the same frailties and complexities and relationships as the rest of us.

And Prince William and Harry have had to grow up without one of the most precious human bonds: the one between a mother and her sons.

The princes looking at photos of their mother in the documentary. Credit: ITV/Diana, Our Mother: Her Life and Legacy

In this new documentary, Prince Harry talks of how it became "normal" growing up without a mum.

He speaks of his inability to cry about his loss (once at the funeral and about one other time, he said).

And both Princes speak movingly about the day they heard their mother's voice for the last time.

We all know that on that August day in 1997, William and Harry were at Balmoral with the Queen and Prince Charles.

Diana called her sons from Paris.

Like many 12 year olds, Harry says he was not someone who enjoyed talking to his parents on the phone.

So he says that it was with some reluctance that he came to the telephone after being called over.

Diana with Prince William while pregnant with Prince Harry. Credit: ITV/Diana, Our Mother: Her Life and Legacy

The Duke of Cambridge also spoke about that phone call.

Asked if he remembers now what Diana said to him during that phone call, Prince William replied: "I do".

Diana died a few hours after the phone call ended.

Neither prince has ever shared that moment publicly before.

And it's clear that the regrets are still with them.

And listening to them talk of how happy they'd been playing at Balmoral in the hours before her death, you can better understand why it was the Queen took the decision - unpopular as it was - to keep the boys in the Scottish Highlands in the immediate days afterwards.

William and Harry on a picnic bench. Credit: ITV/Diana, Our Mother: Her Life and Legacy

Above all else, it is a very personal moment the Princes shared.

Who else wouldn't wish the phone call had been longer?

Or that they should have said something they didn't?

But as William told us just before we watched the film, neither he nor Harry is going to speak like this again.

Which makes this documentary a compelling watch, yes.

But for the Royal Family in general - and for Diana in particular - these interviews are also historically significant.