Amazing Grace is a dull film on a big topic. The fight against slavery began, says the Wikipedia, in 1783. The film starts in 1797 and there's soon a flashback "15 years" at which time (1782) Wilberforce first meets Thomas Clarkson and Hannah More.
The screenwriter, Steven Knight, apparently "read English literature at University College London" so we must trust he's better informed than the Wikipedia, which provides dates that do not tally accurately with those used in the film. It says that "while he was in Cambridge, in 1785" Clarkson "entered a Latin essay competition" with a piece titled 'Is it lawful to enslave the unconsenting?'.
Other writers got busy. In 1788, William Cowper wrote two poems: 'The Negro's Complaint' and 'Pity for Poor Africans'. Cowper's first book, Poems by William Cowper, of the Inner Temple, Esq. (1782), includes a preface by John Newton (played by Albert Finney in the film). The two had worked together on Olney Hymns (1779) after Cowper suffered a psychotic episode and, in despair, turned to god.
Posters for the movie include an extraordinarily misleading subtitle: 'One Man's Fight to Change History'. Even the film explicitly shows that Wilberforce was asked to take on the cause by his political ally, William Pitt (the younger). Clearly this movement, which started with a group of concerned English Quakers, was a shared destiny for many.
The real heroes are more likely the novelists read by girls such as Barbara Spooner (played by Romola Garai, like Ioan Gruffudd, who plays Wilberforce, suitably handsome). Such as Samuel Richardson. A printer with a lucrative government contract, Richardson published three highly influential works between 1740 and 1753.
It is Spooner (aged 20 in 1797) who says the most compelling things in this film, such as when, in a sitting room alone with Wilberforce, she tells him she would, at the age of 14, tell her friends not to eat sugar because "each spoonful contained the actual blood of a slave".
Spooner was two years older than Jane Austen, and would have read the same books. Austen's dislike of Whigs and 'radicals' of any kind (evident in Mansfield Park as well as her letters) was due to her natural fear of revolution.
This fear is expressed by Wilberforce in the movie when, toward the end, he tells Thomas Clarkson (played by a suitably earnest-looking Rufus Sewell) never to speak of revolution in his presence again.
The evangelicals who pushed against slavery were not revolutionaries but Christians who believed that every person is equal under god. It is this that Austen admired in Cowper's work (apart from the fact that Cowper was a very good poet).
The film is not all earnest posturing and undried tears (though there is a lot of both). A good scene comes at the end, after the war against the French has been won, when Lord Tarleton (played by Ciaran Hinds very well) senses something wrong in a seemingly-routine bill being introduced by an uncontroversial member of parliament. What sets him off is Clarkson entering the gallery above the pit. He runs out of the house seeking to gather his party members. But they've gone to the races.
I also must admit to feeling a warm glow when Hannah More (played by Georgie Glen) entered Wilberforce's house. More wrote plays with a puritanical cast and, though no great name now, was influential in her time.