After two prestige television series and a brace of Blanchetts exploring the life and times of Elizabeth I, it's probably overdue for the focus to be turned back on her mum and dad.

They, as those who know their Tudorbethan history will be aware, were Henry VIII and Anne Boleyn, his second wife and the first to lose her head three years later in a marital dispute.

Considerably less well-known is that Henry bedded and impregnated not only Anne, but also her elder sister Mary, the first of the Boleyn girls to become the Royal mistress, giving him both an illegitimate son and daughter before his attentions were distracted.

Indeed, were it not for historical bodice ripper novelist Philippa Gregory, Mary might have remained a forgotten footnote, eclipsed by the legacy of a sister whose refusal to have sex before marriage led to Henry's divorce of Katherine of Aragon and the break with Rome and Catholicism that changed England forever.

However, Gregory's best-selling The Other Boleyn Girl has not only been turned into an acclaimed BBC drama but now, four years later, a lavish big screen version, directed by
 
Justin Chadwick of recent Bleak House fame, and adapted by Peter Morgan who, appropriately enough, was also screenwriter for Granada's Henry VIII two-parter, not to mention recipient of an Oscar nomination for The Queen.

"Before I wrote the novel, hardly anyone knew about Mary Boleyn," says Gregory, whose recently published sequel, The Boleyn Inheritance, picks up the story with Katherine Howard following in her cousins' footsteps.

"She contributed nothing to the historical record, but I saw her story as a contrast between sisters and a parable for the way women make use of their opportunities."

It's also rare in offering not one but two very complex and dramatically meaty female roles. The TV version cast Jodhi May and Natascha McElhone as Anne and Mary, but, inevitably, a bigger screen and bigger budget demanded bigger,

more internationally bankable names. Enter Oscar nominated Natalie Portman and four time Golden Globes nominee Scarlett Johansson as the rival siblings with Australia's Eric Bana as their favours sharing monarch.

But, if the headliners aren't English, the support cast is firmly peppered with homegrown pedigree, including Mark Rylance and Kristin Scott Thomas as the Boleyn parents, rising star Jim Sturgess as ill-fated brother George and David Morrissey the girls' scheming uncle, the Duke of Norfolk.

And, at a time when producers increasingly favour cheaper Eastern European stand-ins, the locations are authentic English heritage through and through, the likes of Great Chalfield Manor, near Bath, Dover Castle, Knole House (Henry's own former hunting Kentish lodge) and, up in Derbyshire's Peak District, North Lees Hall, the Dovedale countryside and Haddon Hall, one of England's finest Tudor houses, all affording splendid authenticity.

It goes without saying that, in other areas of authenticity, the adaptation has taken a few liberties with both Gregory's book and the historical time line; Mary was, after all, only 14 when, pushed forward by her father and uncle to advance their ambitions and status, she became Henry's mistress. However, as Portman points out, history itself is a notoriously unreliable witness.

"In researching I was continuously reminded how biased history is and how it's always someone version of what happened, some sort of fiction. There are so many agendas behind the depictions of Anne, whether portraying her some early feminist or showing her as this witch who put a spell on the King. So, it was good to be able to present our film as an imagination of the events."

But, as Johansson (who, about to star in Mary, Queen of Scots, has clearly caught the costume drama bug) is keen to stress, it's not some 'men in tights masterpiece theatre'. "Justin wanted to find the human quality of the story," she says. "Not just the melodrama of the rivalry but the humanity."

Chadwick happily acknowledges that while there's sex, rivalry, jealousy, ambition, and scandal to rival anything the current monarchy can offer, at the end of the day, it's a tale of two siblings.

"Anne and Mary do some terrible things to each other, but ultimately, they're sisters," he says.

"Sibling relationships are complicated," agrees Johansson, who has both an older sister and two brothers. "Everyone can understand that jealousy and competition. The bond is very strong; only your siblings can read you so well and know instinctively how you feel. The Boleyn girls are two halves of the same person. I think that's always true of sisters of a similar age, even if they don't always want to admit it.

"What Mary admires and is repulsed by in Anne are traits she wishes she had herself. Similarly, Anne comes to realise at the end that she wishes she had some of Mary's traits."

Two halves they may be, but, as seen here, they're also polar opposites in their ambitions and whenever one becomes the more successful in capturing the King's affections, her sibling rival becomes the 'other' Boleyn girl.

"Anne had a sense of self-respect that was uncommon for a woman of her time," offers Portman who, at 26, is actually three years older than her screen sister. "She thought she deserved a status she was not born with, and this ultimately led to her demise. Marriage then was not about love; it was about uniting families to increase their power. Anne accepts this, but then she unexpectedly finds Henry a charming, handsome and educated intellectual companion, and her way of attracting his attention is to challenge him."

"Anne totally buys into the whole competition," Portman continues. "Whereas Mary chooses a way to be happy without life in the court, and ultimately wins by allowing Anne to have the victory that destroys her. It's a family story, with love and intrigue, about children who are corrupted by a world which pushes them to compete rather than support each other. Mary, the survivor, is the one who rejects that world."

When you boil it down, as pawns in a game of male ambition, the sisters are essentially pimped off by their family in the pursuit of wealth and power. Put it to the two stars that, as modern women, this must have seemed terrible, they aren't so sure things have actually moved on to that great an extent.

"I'm grateful to have grown up in a society where my parents encouraged my sister and I to be our own persons and we were never limited because of our gender," says Johansson. "But it would be silly to assume that doesn't happen in other parts of the world."

"I think it's interesting that women have made a lot of headway," laughs Portman. "In that period they were made to have sex or marry for wealth and position, whereas now they choose to have sex and marry for the same things. But (she continues more seriously), you can still see today vestiges of those societal limitations on women. I tend to think that's to do with the opportunities that are available and that sometimes that's the best way for a woman to advance herself. Look at the United States and the number of female CEO's or members of Congress do you see? It's still the dark ages!"