William on Diana. In his own words

Hundreds of programmes have been made about Diana, Princess of Wales, in the 20 years since her death.

Hundreds of books have been published (and in recent months updated) about her character, her personality, her family.

None, however, has been made by her two sons.

Which is what makes the ITV documentary which airs tomorrow night a very unique document of Diana's life.

Princes William and Harry would not have given their consent to take part in a documentary they could not endorse.

And so you should not expect an unsanitised examination of Diana's life.

The programme is just as Prince William described it to us: "A tribute from her sons to her."

So when at Kensignton Palace William stood in front of reporters just before we watched an advance screening of Diana: Our Mother, it was clear that he was happy with the programme and how it portrayed his mum.

"We've never actually spoken so publicly about her," the Prince told us as he explained the reasons why he and Prince Harry decided to do the film, "but we felt that this was the right time to do it."

But then, the Duke of Cambridge added: "We will not be doing this again."

And that is what makes this 90-minute documentary very significant.

It has been, William said, "quite cathartic" going through the process of making the programme - although he admitted it had been "daunting" at the outset.

But the process of going through old photographs for the television cameras was something he described as "refreshing".

Credit: Diana, Our Mother: Her Life and Legacy

It also forced William and Harry to bring to the front of their minds, the memories and recollections of a period in time more than 20 years ago.

William was 15 years old when he lost his mother.

Harry just 12.

William says it is a film made by "those who knew her best".

And both he and his brother, he said, wanted to share with other people the "happiness and the warmth that we had with her".

And as he walked out of the room, it was clear that both Diana's sons are content that the programme does exactly that.