Branding him "a wife and child abuser and a serial killer" in The King's Curse author note, Gregory argues that Henry has been overly romanticised and was, in truth, far from the svelte, charismatic figure portrayed by Jonathan Rhys Meyers in the HBO television series, The Tudors.
"People have this view of him as being this rather eccentric, big, fat person who eats a lot and has six wives," she says. "But one of the reasons why I care about this novel so much is that it's a real correction of this view of Henry either as really sexy and handsome as in The Tudors or this jolly, avuncular, merry king of England. I believe he was a psychopathic killer and even when we teach him in primary schools, we've got to stop promoting this idea that killing two wives is funny or acceptable. We're talking about terrible abuse of his kingdom and powers in someone who, with life or death decisions over six wives, abandoned two to their deaths and executed another two. That's not, in modern society, the sort of behaviour we should be regarding as amusing or entertaining as a kind of morality-free zone."
A member of the rival Plantagenets, Margaret Pole is haunted by the imprisonment at a young age of her brother Edward in the Tower of London by the former king, Henry VII. Seen as a threat to their rule, she is married off to loyal Tudor supporter Sir Richard Pole, the Governor of Wales. When Henry VIII ascends to the throne, she is appointed chief-lady-in-waiting to his first bride, Catherine of Aragon, but finds herself in an increasingly perilous situation, as he grows increasingly unstable. Eventually at the age of 67, she gains the dubious honour of becoming the oldest person to be put to death on Henry's blood-soaked scaffold.
"It's a bit of a spoiler from the point of view of the book since we're exploring her life," admits Gregory. "But it's probably her best-known claim to fame. When I was writing the book, it seemed more and more that it would do her a disservice if we were to see her as just a victim. What you don't see is her incredible courage and ability to survive. In a sense, death is the last thing that happens to us all and for her it's the last, worst thing that happens in a long life.
"She enters a family of courtiers and is the mother of Reginald Pole, a really famous Catholic theologian, who ends up leading the resistance to the Reformation and Henry's move away from the church. He comes back to England with Mary I, becomes a cardinal and restores what they regarded as the true religion. So it's a family that is really, absolutely at the heart of power. She loses at one point but that's not the end of the story of the Poles at all."
The last in a sequence of six novels that make up The Cousins' War, The King's Curse brings the story of the Tudors and the Plantagenets to a definite conclusion. "It is kind of like never say never," laughs Gregory. "But this book brings us up out of the Plantagenets and into the Tudor period, so it's the last in the series in that sense. After this, I might do some of the other Plantagenet women but the books won't have that sweep. This series started chronologically with The Lady Of The Rivers, which was about the Duchess of Bedford, and has just walked through these extraordinary women's lives to this woman, who survives for so long before she eventually dies under Henry's axe."
With the first three books in The Cousins' War forming the basis of the BBC television series The White Queen, Gregory reveals that American network Starz is planning to adapt the remaining trio as The White Princess.
"I was very pleased with it as I thought it looked outstandingly beautiful and the actors' performances were great," she says of The White Queen. "Of course, there were some things about it that I would have preferred to have been more like the books, but when you go from three books to 10 hours of television, there are inevitably huge compromises between what will and what won't be in it. But by and large they got it right."
The King's Curse (Simon & Schuster $36.99) is out now.