When a reader brings Philippa Gregory's attention to an inaccuracy that has slipped into her historical fiction, Gregory will, she says, promptly pick up the phone to her publisher and get it tweaked for the next edition. "They must get sick of me here," she says, flicking her eyes up at the Simon & Schuster office where she sits, for the tweaking and fine-tuning she has requested over the years.
There are, though, the less helpful letters - the incorrect corrections – that some armchair historians shoot off to her to say she has been too hard on Anne Boleyn or that she has invented Plantagenet rivalries where there were none. Lately, grievances have revolved around the book-inspired-TV serial, The White Queen, based on her Cousins' War series. Some have sneered at its sexed-up version of history but the greatest offence seems to have been caused by the zips on costumes. Zips in the 15th century? The television critics have had a field day, which has irritated Gregory all the more, though she was an executive producer and had no part in writing the script. Still, facts are facts.
"There are no zips in it," she says shaking her head, ringlets tremulous. Another charge that she played up the rivalry between Elizabeth Woodville and the Earl of Warwick is also unfounded, she adds. The rivalry is well-recorded. "Many people would say it's the reason for Warwick's rebellion. People say it's not real but I don't know where they've got this opinion from… It's always horrible when people are critical, but especially if they are mistakenly critical," she says.
She appears surprised to be enjoying the series herself, currently showing on BBC One, as difficult as it was to surrender creative control. Her input was welcomed – she looked at the rushes (first prints of filming) and recommended changes - but the final say was not hers. "If you want to change something, you have to convince three or four people. Now and then I would say that I couldn't stand something and they listened to me. With every script, I would probably offer about a dozen notes with things I thought should be changed, and about two-thirds of them would be. The worst problem for me was that they would sometimes feel that to make the action of the film clear, there would be an invented scene and I would say 'it's not in the history, nor in the novel'."
Gregory, 59, lives and writes on a North Yorkshire Moors farm (there are photos of pet ducklings on her phone) and for the past decade, she has undertaken intensive research on the 15th and 16th centuries, an exercise made easier by her background – a history degree from Sussex University followed by a PhD from Edinburgh on the 18th-century novel. After six books on the Tudors, she began her Cousins' War series which goes further back to the Plantagenets and the War of the Roses. Her latest, The White Princess, is about Elizabeth of York, who is forced to marry Henry Tudor after he defeats the House of York to take the throne as Henry VII. "I read for a year before I begin to write a thing. There is always a point in the research with all my books when I say 'this is huge'. I'm daunted by the amount of material I have to understand, the amount of books I have to read… Then I just start, and I get so interested that I really start to get a handle on the characters…"
She has followed this method for the past 26 years (and 32 books) and it has worked wonders for her: phenomenal sales, famous fans (Sarah Jessica Parker, Vera Wang and the Duchess of Cornwall whom "I send a book to every year"), and the satisfying symbolism of a boardroom full of books at Simon & Schuster on the day we meet, waiting to be signed.
Gregory was a rebel at school, getting an "E" for A-Level History and going to Sussex University to read English literature. It was only when she sat in on a history lecture that an inspirational tutor fired her passion. She converted her degree immediately. The lecture was by Dr Maurice Hutt, who has, Gregory notes with eyes filling with tears, just died.
"His obituary was in The Times," she says, regaining her composure quickly. "The first time I saw him, he was fierce. He asked the class 'What do you think you are here to do?' and then said 'I'm here to do research. You may watch'. I thought 'research! That's what I want to do.'"
Some have credited Gregory for sparking our contemporary obsession with the Tudors, equal only to the Victorians' insatiable appetite for them. She wrote The Other Boleyn Girl in 2002, years before Michael Hirst's The Tudors became a hit TV series, and before Hilary Mantel won her Man Booker plaudits.
"There had not been this explosion of material. [Initially] I was going to do a book on Tudor piracy. I was reading the records of the Tudor navy when I came across Mary Boleyn and thought, 'who is she?' I looked and looked for her and found she was Henry VIII's lover and Anne Boleyn's sister. I thought, 'Why haven't people written about this? It's enormous.' We knew the mistresses - it's a well-ploughed field – but no-one thought they were interesting. Since that novel, there have been four biographies of Mary Boleyn."
Her passion for historical fiction has also been fuelled by the urge to imagine histories from a female perspective, which sometimes presents its own problems for its dearth of primary source material; the lives of the women who accompanied powerful men-of-state have often remained unrecorded. "The history of the history of these women is very interesting. If you read women's history of this period, you have to read between the lines. There is one biography of Anne Neville [of The Kingmaker's Daughter]. There is no biography at all of Jacquetta of Luxembourg [the mother of Elizabeth Woodville, and protagonist in The Lady of the Rivers]. You are trying to work with facts that are not established. It's [a process] like detection when you are dealing with someone who is simply not recorded.
"I'm interested in what power there was for these women. They were not powerless. [In The White Princess] Elizabeth is apparently completely powerless. Then she gets her own allowance and she is also innately a powerful woman because she has been raised as one… And Henry VII's mother [Lady Margaret Beaufort] commissioned [historian] Polydore Vergil to write her history and told him what to say. She understood the power of propaganda. She was a very devout woman who would rather have been a nun than anything else and yet she invented a title for herself - 'My Lady the King's Mother.' That's grandiosity."
As historical fiction has seen its popular 21st-century ascendancy, so debates around its navigation of factual history have intensified. The historian David Starkey recently suggested that such fiction has no historical authority, and perhaps added unintentional insult to injury by calling Gregory's novels "good Mills & Boon".
"I had no idea he was an expert on Mills & Boon. I'm surprised," she quips and the topic is promptly dismissed. But a discussion on the boundaries of historical fiction is a cause for more serious reflection: "Of course you mix history and fiction. It's called historical fiction. Everybody knows what this thing is. It's not as if we have just invented the genre. Of course, anybody can absolutely say they don't like it."
No fact - or its creative distortion - is off-limits to the historical novelist, she believes: "There is supposed to be invented stuff, and there has got to be invented stuff. It would not be a drama or a novel otherwise but a documentary or a history book."
Yet there is a disjunction between her principles and her practise, she admits. She reads almost no historical fiction – "I don't like it. It's no use to me" –and she abides by particular standards that don't correspond to the general principle. "There is not a historical fiction police that comes around and knocks on your door if you mess around with the facts and I don't believe there should be rules. On the other hand, I impose on myself my own rigorous rules and the fiction has to meet my satisfaction. So I don't change facts for my convenience. I create dialogue and I create internal thoughts, but I write a novel based on psychological 'truths'."
She has latterly become immersed in the Plantagenet period which led to Tudor rule, and even after writing five books on this period, with all its murderous ambitions and political machinations, she has not tired of writing about absolute power in royal courts. So it is a surprise, given this passion for British monarchy, that she is not a royalist, nor a follower of the House of Windsor, with its weddings, new babies and the rest.
"I'm a republican. I'm not interested in modern-day monarchy and I think a monarchy does not have an effective place in a democracy. The thing that interests me with the Tudors and the Plantagenets is the intersection between political power and personality."
'The White Princess' is published this week by by Simon & Schuster (£20)