18/06/2014 13:36 BST | Updated 18/08/2014 06:59 BST

A New Way to Talk About Slavery

Much has been made of how the two girls in the Zoffany painting inspired my writing of Belle. As important to me was the invisible third person in the painting, the man who put them there in terms of such equality: Lord Chief Justice Lord Mansfield. From the outset I went in search of the historical Lord Mansfield and I found him between two judgements.

In 1759 at the height of the slave trade he wrote an opinion that has echoed down through the centuries. It remains astonishingly relevant today. As recently as 2008 it was cited in the US Supreme court decision in Boumedienne vs. Bush. This opinion severely limited the reach of Habeas Corpus declaring that it does not extend to off shore territories. This dry practical opinion was necessary for the orderly functioning of maritime trade. Because of it mutinous Sailors held in brigs and slaves could not avail themselves of Habeas Corpus. Because of it today, neither can prisoners in Guantanamo Bay.

13 years later in 1772 he made another judgement. A judgement so radically different that it might have been written by another person. In language both astounding and passionate he extended the reach of Habeas corpus to every slave in England. "The state of slavery is odious, that nothing can be suffered to support it, but positive law. Whatever inconveniences, therefore, may follow from the decision, I cannot say this case is allowed or approved by the law of England; and therefore the black must be discharged." With these words he tore up the rule book of order. "Let justice be done, though the heavens may fall."

Lord Mansfield made that journey from order to justice. Finding the reason for that change gave me the character of Lord Mansfield, charting the course of that change gave me the story of the film. This led me finally to a desperately needed new way of discussing slavery.

For Lord Mansfield in 1756 the purpose of the law is clear - order.

Then sometime between 1761 and 1766 he took in a little mixed race child, his great niece, Dido. Not out of goodness on her part or courage on his but out of that same sense of duty that guided his life. He took her in on his terms. Freedoms must be sacrificed in the interest of order, whether aboard ship or round his dining table.

And then she changed him. It is not wishful thinking on my part. In 1759 he was already Lord Chief Justice, he was already living at Kenwood, he was married to the same wife. He already had two other nieces living with him - older girls including Lady Mary Murray. The only big change between those two judgements was Dido Belle.

He may have kept her out of duty, but the increasing comfort he kept her in, spoke to me of grow-ing love. The few historical records show me Dido was special to him. Not just the silk bed hangings and asses milk when she was ill. Of all his adopted daughters it was Dido he taught to read and write beautifully, Dido he spent time with as a companion. He taught her to love the things he loved. She mattered to him.

How did that love affect his journey? My choice of story had a personal beginning. The only little black girl in the class, cringing because of how slavery was always spoken about. Well meaning white teachers telling curiously detached stories of unspeakable black suffering and humiliation and helplessness - a black woman hung upside down with her bottom exposed, a pregnant woman flogged to death. These stories were interspersed with tales of the white people who "saved" them - Quaker women perhaps, or the great white abolitionists. Wilberforce. Newton. All named and revered. The violence of slavery was confined to cartoonish pirate captains. The towering achievement of that great black abolitionist Oluduah Equiano reduced to "celebrity freed slave". These lessons left me ashamed and everyone else exonerated.

Belle was going to be different. Shame belonged to a cultured man living in Kenwood. And change belonged to the mixed race girl history had given us. A girl of learning and beauty who could have agency in her own story. So I set the script when Belle was 17 and wrote a love story with a passionate young abolitionist, John Davinier.

My choice of the Zong insurance claim took responsibility for the suffering out of the hands of the Captain Hook and his minions in their nameless slave ships and brought it home to the people who were truly responsible for that trade. Bankers, Insurers, everyone who profited by it. Everyone.

Through her relationship with the Davinier, Dido comes to understand who she is. In Dido's grow-ing love for herself she shows Lord Mansfield what the right judgement should be. Lord Mansfield's love for her dared him to do what was right, and in the end that love gave him the courage to do it. And in doing right he accepts his earlier guilt. The shame is his, not mine.

Some relate to Belle in terms of emotion, seeing their fathers and mother in laws in the wonderful characters I created out of scholarship. Brought up by a family of lawyers just after Independence, Lord Mansfield's central dilemma was familiar to me - "What is the purpose of the law ?" - was discussed as many times around the dinner table at home as it is in the film. I could say the filmic Lord Mansfield was based on my Dad too. But we must not lose sight of the fact that these characters and stories rose from research into the historical record and looking at it in a different way.

This journey was made by the real Lord Mansfield. That it is true is very very important.

It gives us a new way of discussing slavery, where guilt is properly apportioned and the agents of deliverance properly recognised. And in the end, we all end up on the same side. The side of right. the side of love.

A new abolition story we can all cheer.