Lord Mansfield, the great 18th-century lord chief justice, is an unlikely cinema hero.

This must be a first: from this weekend, moviegoers in the US get to see a history blockbuster in which an 18th-century British lord chief justice is one of the heroes.

Amma Asante’s Belle tells the story of the relationship between William Murray (later Lord Mansfield) - who as solicitor general, attorney general and especially lord chief justice, bestrode Georgian England’s legal scene for five decades - and his great niece Dido Elizabeth Belle.

The drama arises from the fact that Dido Elizabeth is a ‘mulatto’ – the daughter of Murray’s sea-captain nephew and a West Indian slave. 

Belle opens in the UK next month, so my supposition that Mansfield LCJ is one of the good guys comes from the fact that in real life he provided for Belle handsomely in his will and, of course, ruled in the Somerset case, widely held as a landmark decision in outlawing slavery in Britain. (Another clue is that he is played by Tom Wilkinson, the gay judge tortured by a terrible personal secret in The Best Exotic Marigold Hotel.)

The real-life Mansfield made an unlikely people’s hero, however. In 1780 his Bloomsbury home was razed to the ground by the Gordon rioters, apparently stoked by his apparent tolerance of Roman Catholicism. In my own business, he is best known as the persecutor of John Wilkes, martyr or scurrilous journalism. His ability to amass a fortune from wheelings and dealings while a public servant, though standard practice in the 18th century, raises eyebrows today. 

Fortunately a thorough but very readable new biography* by Norman Poser, professor emeritus at Brooklyn Law School, puts this giant of jurisprudence in historical and legal context. The picture that emerges is not of a nice man, but one with a finely honed sense of justice, whatever the consequences. His ruling of the right of French and Spanish ship-owners to insure their vessels in London despite their nations being at war with Britain, would no doubt prompt tabloid comment today.   

Likewise his treatment of slavery, which, by Poser’s account, is legalistic nit-picking at its best – or worst. In the second half of the 18th century, slavery’s legal status in England was ‘confused and contradictory’. While Blackstone’s Commentaries on the Laws of England (1765), stated that ‘a slave or negro, the moment he lands in England, falls under the protection of the laws, and with regard to all natural rights becomes... a freeman’, a thriving market in recaptured slaves suggested that practice was otherwise. 

According to Poser, Mansfield was ‘reluctant to make any decision that would call into question the legality of the slave trade or slavery’. The Somerset case was forced upon him: it concerned a writ of habeas corpus against a merchant, Charles Stewart, on behalf of the escaped slave James Somerset.

Mansfield’s initial - and hardly enlightened - solution was that Somerset’s abolitionist protectors buy him and set him free. But both abolitionists and slavers wanted a test case and in 1772 he had no alternative but to rule on the narrow question of whether colonial slavery laws could be enforced in England. 

His judgment to a packed courtroom has gone down in popular history: that the state of slavery ‘is so odious, that nothing can be suffered to support it, but positive law’ and no such law existing, ‘therefore the black must be discharged’.

Many people first hearing the words thought Mansfield had put an end to slavery. In fact, Poser says, Mansfield ‘had made the fine distinction that, while slavery was not illegal in England, the court would not recognise a slave owner’s dominion over a slave’.

Eleven years after his Somerset ruling, Mansfield sat in a more notorious case, concerning the throwing overboard of 150 slaves from the Liverpool slave ship Zong, en route from West Africa to Jamaica. The Zong’s insurer refused to pay up for the slaves thrown overboard, and the ship-owner sued.

A jury sided with the ship-owner, on the grounds that loss was the result of the normal perils of the sea.

On appeal, Mansfield ordered a new trial because the ship-owner had not demonstrated that it was necessary to throw the slaves overboard. Poser notes: ‘Nowhere in his short opinion was there any suggestion that the captain and crew of the Zong were murderers.’

I’ll be fascinated to see how Belle deals with that.  

*Lord Mansfield: justice in the age of reason, Norman S Poser, McGill-Queen’s University Press

Michael Cross is Gazette news editor