'It's the definitive account of Diana's life as she saw it'
To mark 20 years since her death, Channel 4's controversial programme on Sunday will air interviews the Princess of Wales had in the early '90s with her voice coach Peter Setellen. Francesca Gosling speaks to the producer and contributors about the hotly debated documentary.
With a fairytale royal wedding, a soap opera love triangle and a tragic death that broke hearts around the world, Princess Diana's story is still one of the most fascinating and scandalous in UK history.
But while numerous documentaries have been made about her extraordinary life - most recently including first-hand accounts from her own sons William and Harry in ITV's ground-breaking programme last week - how much of the truth do we know?
Now, to mark the 20th anniversary of her death in a Paris road accident at the age of 36, TV viewers will hear in her own words what really went on behind the palace doors.
As her marriage to Prince Charles broke down, Diana spoke candidly about her heartache and the desperate measures she took to save the relationship to her voice coach, Peter Setellen.
On Sunday, the filmed interviews will be publicly aired for the first time in a feature-length documentary by Channel 4 that promises to surprise even her most dedicated fans.
With contributions from former confidantes - including private secretary Patrick Jephson, personal protection officer Ken Wharfe and ballet teacher Anne Allan - the footage finds her between 1992 and 1993, shortly before her famous BBC Panorama interview.
"This is the definitive account of Diana's life as she saw it," says producer Charles Furneaux. "And it would be very hard for anyone to supersede it.
"We hope it will be the resource that people come back to when revisiting the story of the Windsors. It will give viewers an insight into the process she went through as an innocent and inexperienced young woman, suddenly launched into this strange world."
Ralph Lee, the broadcaster's head of factual, describes the tapes as a "treasure trove" of footage marking a period in royal history as significant as Edward VIII's abdication.
After months of persuading a "reluctant" Setellen to relinquish licensing rights to the videos, he says: "For those of us who grew up watching her on the telly, it turns on a new light and illuminates her in a completely different way.
"Like discovering a family album you didn't know you had, you go back through the same events but you see images you hadn't noticed before."
Diana tells her trusted interviewer everything from the "best time of (her) life" as a self-proclaimed "rebel" living with girlfriends in Earl's Court, to Charles' bold courting technique which involved him "all over me... following me around like a puppy".
After marrying the heir to the throne at barely 20 years old, she tells of her realisation that her husband's heart belonged to another woman and how even the royal family's "top lady" failed to offer much in the way of support.
Viewers will see how her isolation drove her towards an eating disorder, a deeply loving (but, she insists, non-sexual) relationship with her then protection officer, Barry Mannakee, and, ultimately, to earning her People's Princess nickname as she channelled her energy into championing taboo health and social issues.
"Her narrative is almost an archetype of marriages that many people go through and I think in many ways it's a woman's film," says Furneaux.
"A lot of women will identify with aspects in the story and how Diana always made it clear that she was not going to be pushed around and that she was always going to fight back - that attitude is what is still so impressive about her."
While the long-running relationship between the Prince of Wales and Camilla Parker Bowles was public knowledge throughout his union with Diana - prompting her to summarise that "there were three of us in that marriage" - Jephson maintains that it was the key reason why some sought to demonise the young princess.
The former monarchist says openly in the film: "What I loved about the monarchy died with Diana, for me."
He accuses Clarence House, Charles and Camilla's London residence, of encouraging public enthusiasm for their relationship through a campaign painting Diana as "inadequate, unsuitable and unworthy".
"If there is popularity, it has been the result of a sustained and systematic campaign involving the finest PR people money can buy," he insists.
"Like many, sometimes unscrupulous PR operators, they make their client look good by making the opposition look bad.
"But people eventually tend to see through spin, and Diana's authenticity is one of the reasons that we are still talking about her 20 years after her death."
But the story is not all sad.
As well as describing the 180-minute edit as a demonstration of Diana's "backbone of steel", Jephson says: "What you get beautifully through those tapes is her sense of spontaneity and fun.
"She was a fabulously inspirational boss, because even when things were tough - and there many tough days - there was never a day so tough that she couldn't see the lighter side.
"With every cloud she saw a silver lining and it was her rather brave sense of refusing to be cowed by circumstances that meant she could always lift, not just her own spirits, but people around her."
Both former royal employees credit Diana with "changing the face of the British monarchy," breaking its "theatre" to engage with hard-hitting issues of public life and, most importantly, passing that legacy onto her sons.
Recent years have seen William and Harry actively support causes such as disability and mental health, even opening up about the profound effect the loss of their mother had on their own childhood.
With Harry now a similar age to Diana when the Setellen tapes were recorded, Wharfe says: "It's a film that both the princes should see - it shows a side of their mother that they themselves would endorse."
Jephson adds: "If Diana was still alive I reckon she would still be surprising people.
"There was nothing spun about Diana, she was wonderfully unpredictable and her sense of anything being possible is an important element in the monarchy's future viability."
- Diana: In Her Own Words airs on Sunday at 8pm on Channel 4