Princess Diana pictured at a Breast Cancer Research charity dinner at Nina Hyde Centre in Washington in 1996
Which is lucky, because critics of her latest book on Prince William, Born To Be King — serialised in the Mail this week — accuse her of being an insensitive, untruthful, poisonous, hatchet merchant who moonlights as Camilla Parker Bowles's public-relations guru and is probably in love with Prince Charles.
She has written books about both Charles and Diana, and was on their first official visit (to Wales) when Diana was pregnant with William. So it seems natural she should have turned her attention to the young heir.
The eve of his 30th birthday (which falls next month) was, she felt, a suitable moment to release a biography — and she 'was genuinely interested because he seems such a sane man . . . and I wanted to know how he was [that way] with that childhood.'
But it is not the William in her book that has irked people — it is her portrait of his mother.
The Diana whom Junor has described is an unsatisfactory and manipulative mother, prone to tantrums, self-indulgence and jealousy (she sacked nanny Barbara Barnes when William was four because she envied their strong bond).
Junor also reveals Diana was reluctant to warn William, then 13, about her infamous Panorama interview in 1995.
His Eton housemaster begged her to; eventually she agreed to talk to her son, for five minutes, telling him he would be proud of her. He was devastated as he watched it in his housemaster's study.
Penny tells me now that she believes Diana was unwittingly recreating the tension of her own childhood.
After her mother had suffered the early death of a son and the misery of a violent marriage, she left with her lover, Peter Shand Kydd, when Diana was six.
'It meant that Diana didn't know how to be a mother. She lost hers at an early age. And Diana had a serious mental illness. Bulimia is not just about trying to get slim. It's all about control: the one thing you can control is your diet.
'Diana's early life was chaos. She was dishonest [with others] because that is part of the disease.'
Hence, she didn't think the Panorama programme would upset her children, with its revelations of 'three of us in this marriage' and her unhappiness with Charles, because she simply couldn't see it from anyone else's point of view.
This week, Junor critics have pointed out that Charles gave his own TV interview to Jonathan Dimbleby before Diana did Panorama. So shouldn't Charles take the blame for the television confessionals?
'He never criticised her. She, on the other hand, said Charles was a bad father, bad husband and shouldn't be King, so she took it up a notch,' Junor responds.
The general objection to her book is: how can she 'applaud two horrid adulterers' — as one disobliging comment describes Charles and Camilla — at the expense of Diana?
'She wasn't speaking to her own mother at the time she died.' Junor concedes that Diana could also be very loving: 'She could be expansive, enchanting, demonstrative . . . she could light up a room.'
This was the sunny Diana who first met Charles. According to Junor, the moment Charles proposed, Diana turned into an unhappy creature and he blamed himself.
Charles, Junor insists, is 'a deeply honourable person who loves his children'. Yet she is adamant that she is not his spokeswoman or ally.
Coming in for criticism: Penny Junor photographed at the Dean Street Town House, in Soho, London
As Junor tells me: 'He grew profoundly depressed when his marriage wasn't working.
'The man I interviewed in 1987 was already deeply unhappy. He wanted that marriage to work, he wanted what his friends had — a happy family.
'He wanted someone to support him, to help him, to be with him and boost his confidence, and he felt personally responsible for the failure of the marriage.
'Camilla is a red herring; he didn't start seeing her again until Diana had been unfaithful [with her bodyguard Barry Mannakee in 1986, before James Hewitt].
'People talk about this big love affair between Charles and Camilla, and yes, it had been in the Seventies, but he knew that wasn't the way forward.'
However, he was 32 and Diana was just 20 when they married, so wasn't he more responsible for the mismatch?
Junor replies Charles was 'a vulnerable character. He always felt undermined by his father, and his mother had never been very affectionate.'
Princess of Wales and retired US General Colin Powell share a smile while attending the United Cerebral Palsy annual dinner in New York in 1995
Philip, Junor believes, meant that he must decide because it wasn't fair to keep the media attention on Diana if he wasn't serious.
But Charles took this as an instruction to propose, and so he did.
'Until he asked her to marry him, Diana was the most delightful, bouncy, funny, charming girl who seemed to love everything about him.
'He thought: “I might not be in love with her just yet, but I easily could, because she is adorable and everyone adores her.” On paper, she was perfect. She was the daughter of an Earl, her father had been an equerry and lived on the Sandringham estate, and the Spencers and the Royal Family had intermingled over centuries.
'But the minute they got engaged, she turned into a stranger — he was committed and thought the problems were him and the stress of the engagement.
'And he was a coward.'
Diana, the Princess of Wales, during her visit to Leicester to formally open The Richard Attenborough Centre for Disability and Arts in 1997
'It was no picnic for him,' Junor says. 'He lived with a sense of failure for years.'
She accepts that her critics will never agree with her belief that the Prince didn't stray, nor even intend to, until the marriage had irretrievably broken down. 'I wasn't in the room with them, but that is what I think,' she says.
It might have been a perfect match on paper but, in reality, joining the Royal Family could hardly have been worse for the fragile Diana.
She had been brought up not only almost without a mother but 'without discipline. She'd never been made to stick at anything — finishing school, dancing classes... she could give up whenever she liked. 'And the Royal Family is all about discipline.'
Given this, it is remarkable that William and Harry have turned out as well as they have.
Two things, says Junor, 'can send children off the rails — a lack of love, and loss'.
She believes the loss of nanny Barbara Barnes and then Diana had a dreadful impact on both boys, but the one thing that they never lacked was love.
'They were surrounded by it. One of the strengths of “The Firm” is that there are so many people around to be attached to.
'And they are close to their grandparents.'
'Until he asked her to marry him, Diana was the most delightful, bouncy, funny, charming girl who seemed to love everything about him.'Although the Queen has had a distant relationship with Charles, she is very close to William. 'She stepped in after Diana died, asking him for tea at Windsor Castle, over the bridge from Eton,' says Junor.
His relationship with Philip, too, seems easy and affectionate.
Both grandparents are proud of William and Harry. Junor writes in her book that Harry's wildness is exaggerated and, anyway, a part of military life: periods of intense pressure followed by bouts of keen boozing.
'William was just as bad, but he never got caught.' [Though he never took drugs, as far as anyone knows; and Junor does know because she lives in Wiltshire and hears the Cotswolds gossip.]
Harry may be blessed with a sort of relaxed 'just do it' mentality, but initially it was the other way round.
William used to be the more confident child but, unsurprisingly, things got to him. You have to understand that his childhood spiralled out of control. He has overcome it very well, but is still the result of it.
'Partly because of the difficulties in his parents' marriage, partly because of the approach they took — both spilling their guts to the world.'
Consequently, William has adopted a different approach to the media. He shares little and expects his friends never to talk about him to the Press.
That background, Junor believes, explains his extended eight-year courtship of Kate. 'He had lost women he loved, so he had to test her to see whether she, too, would abandon him.'
The Prince of Wales and Lady Diana Spencer in the grounds of Buckingham Palace after announcing their engagement in 1981
one woman. They weren't always as lovey-dovey as the Press made out. Kate was always the more clingy,' she tells me, 'but also so normal, and that appealed to him, as did her happy family.'
At one point in the book, a tutor claims that Kate is 'just another girl in a pashmina'. But Junor thinks it is precisely this down-to-earth side of Kate that appeals to William. 'A “me, me, me”, more flamboyant kind of girl, or a fashion icon, wouldn't have suited him at all.
'She's steady and normal, although I'm sure she can also be great fun. She seems very well-balanced.'
To a boy who longed for normality, the Middleton kitchen table in Bucklebury, Berkshire, must have seemed like heaven.
'The Firm': the Queen Mother (centre) on her 90th birthday in 1990 with her family. Princess Diana is at the back with Prince Edward
'Yes, she is tough, a bit of a social climber — anyone who has struggled might hope their daughter marries well — but you'd have to be insane or stupid to want to marry into the Royal Family for a nice life.
'A year after he married Kate, William has a wife who is level-headed, absolutely adores him and hasn't had her head turned by fame.'
Interestingly, it was the Queen to whom William turned for advice about his wedding.
'She is wise, decisive and thoughtful,' Junor says. 'She has seen and done everything, and has a calmness.' On that, at least, everyone can agree.
Read more: http://www.dailymail.co.uk/femail/article-2142167/Ive-called-vile-evil-telling-truth-Diana-The-author-royal-book-year-hits-critics-insists-Diana-mentally-ill.html#ixzz1uU78aALX