Despite the success of 1970s teleseries Roots, cinema has dragged its heels on serious depictions of the slave era.

Amazing Grace, the 2006 British picture about William Wilberforce’s abolition struggle, may well have paved the way for the recent Oscar winner, 12 Years a Slave, but it had only one black role, and a minor one at that.

This good-looking British film from black director Amma Asante takes a different approach to both those stories by examining a significant legal case that helped to weaken the slave trade from the viewpoint of a young, black heiress, Dido Elizabeth Belle (impressive newcomer Gugu Mbatha-Raw).

This illegitimate daughter of a sea captain and a West Indian woman is brought up in luxury by her father’s (Matthew Goode) aristocratic uncle – Britain’s Lord Chief Justice (Tom Wilkinson). Her skin colour and illegitimacy mean she suffers the slings and arrows of her adaptive family’s shame. The upside is she has a chance to quietly intervene in the legal case around the Zong massacre, a famous mass drowning of slaves.

Though the story is set up elegantly, by the halfway point it’s obvious where it’s headed and it ploddingly sets about going there.