Headey plays Luce, a north London florist who unwittingly wreaks havoc on Hector and Rachel, a beautiful, seemingly perfect, couple, when one of them falls in love with her after she provides the flowers for their wedding.
Conveniently timed to coincide with the post-Brokeback fascination with portrayals of homosexual romance, our expectations are confounded when we discover that it is Rachel, not Hec, who has fallen helplessly for Luce's charms.
As they await arrival of the bride, Henrietta confides to Luce that she is nicknamed "H" because, supposedly, her mother exclaimed "Jesus H. As Rachel is walking down the aisle, her eyes wander and she makes eye contact with Luce.
'I think the Americans have been a bit disappointed there aren't more issues here', she muses. It's about love and human relationships, and responsibility, and guilt … 'Life would be pretty rubbish without that possibility wouldn't it?
How will the story stack up against the greatest films about business?
Europe has been a place of battles and political intrigue for centuries.
Headey, ever modest, refuses to elaborate on this story, which sent industry gossip-mongers into a frenzy.
'Look, as far as I was concerned, I went to an audition, got a part and turned up on set.
'But, really, it's just a quirky, honest, painful and brilliantly written little story about two people who fall in love, completely out of the blue. and, most of all, it's about timing.' When I ask her if she believes in its premise - namely, that you can catch someone's eyes across a crowded room and everything can turn upside-down - she looks horrified. ' Headey met Parker years ago on Loved Up, a film he had written for BBC2; 'Ol is a genius', she declares.