When a film to mark the 200th anniversary of the abolition of slavery was suggested to him, the director Michael Apted just about stifled a yawn. How worthy, but how dull. Just look at how the risible Chrisopher Columbus: The Discovery and the plodding 1492: Conquest of Paradise turned Columbus into a crushing bore on the 500th anniversary of his discovery of America. Apted, a prolific film-industry veteran, had done everything from helping Sissy Spacek to win a best-actress Oscar in Coal Miner’s Daughter, in 1980, to delivering Pierce Brosnan’s best Bond, in The World Is Not Enough, in 1999. He was not, he decided, going to make a fool of himself over some politically correct nonsense about William Wilberforce’s efforts to stop the slave trade.

Then he met the screenwriter Steven Knight, whose eclectic CV includes the screenplay for 2002’s Dirty Pretty Things and the co-creation of Who Wants to Be a Millionaire? The way Knight saw it, the film would be about young rebels kicking out at society, challenging the Establishment, protesting in parliament and organising petitions. “It sounded more like the 1960s than the late 18th century,” Apted says. “I wanted to make a film to show how heroic and relevant politics can be. I also realised this was a chance to make a British-based film about a British subject, with a British cast.”

The money men, though, were American, and put pressure on Apted to hire a Hollywood star as the lead. He managed to manoeuvre on that one by hiring the Welshman Ioan Gruffudd, 33, famous on American television as Hornblower and on film as Mr Fantastic in the surprise 2005 hit Fantastic Four. The actor is based in Los Angeles these days. “I took a firm stand,” Apted insists. He then picked Romola Garai, 24, as Wilberforce’s wife, Barbara, and Benedict Cumberbatch, 29, as Wilberforce’s friend William Pitt the Younger, who was just 24 when he became prime minister in 1783. Rufus Sewell plays the fiery radical Thomas Clarkson, who in 1787 spent five months touring English ports and compiling evidence of the cruelties of the slave trade.

“Once the script was ready, we had no difficulty in attracting the best names,” Apted says. “But I did not want celebrity casting. I wanted to get believable performances out of people who are well known, rather than international stars.” And he was true to his word. Ciaran Hinds is Lord Tarleton, whose backing of the slave trade was absolute; Toby Jones plays his supporter the Duke of Clarence; Michael Gambon appears as Pitt’s opponent Charles Fox: and Albert Finney plays John Newton, the captain of a slave ship who underwent a religious conversion at the age of 45 and wrote, among many hymns, the words to Amazing Grace.

Background

• Amazing Grace

• Amazing Grace - smaller is better

• Abolition man

• Amazingly graceless remarks by dull critics

• Film row on ‘prettifying’ of slave trade

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Wilberforce, an MP at just 21, brought in his first bill to abolish the slave trade in 1787, when he was 28. It took him 20 years to realise his ambition.

“Britain relied on its overseas wealth, built up by the slave trade,” Apted says. “It was the equivalent of suggesting today that we pack in petrol altogether and never fly again. The majority in parliament and the wealthy Establishment could not even contemplate life without the slave trade. That is why it took so long to bring sense to the situation.”

At the 16th-century house near Guildford used for many of the meeting-hall and parliamentary scenes, Gruffudd, who has been filming a sequence in which Wilberforce is disheartened by the response to his initial bill (it attracted support from just 16 MPs), cheerfully admits he knew nothing of the history. “It has been an education,” he says. “These were the days before mass-circulation newspapers, and there was no pictorial way of showing the horrors of slavery. So news travelled slowly, and they had to rely on speeches to make people change their mind. It really was a youthful revolution, with all the main campaigners still in their twenties.

“You look at the facts, and it makes me feel angry, even today. Slaves were chained together, women and children included, for a journey of thousands of miles. The sick or dead were thrown overboard. Those who had no sale value in the West Indies, where they were used on sugar plantations, were left to die on the wharf. Yet these actions were defended to the hilt.”

If there is an actor who can bring an evangelical flavour to his role, it’s Gruffudd. His parents are committed Christians and, growing up, he attended a Welsh nonconformist chapel. He also won a national eisteddfod folk-singing competition, and when he has to sing Amazing Grace in the film, he is pitch-perfect. His father, a former deputy headmaster, will be relieved. He had some stern advice for his son after viewing Fantastic Four. “I hope”, he said, “to see you again in something with a bit of substance.”

His 6ft son is clearly relishing playing a hero of a different sort, albeit one who was, in real life, only 5ft 4in tall. “I knew the film’s financiers were after someone more recognisable than me,” he admits. “But I think the cast lineup is absolutely right. All the main countries in Europe were involved in the slave trade, but it was Britain alone that took a stand.” The abolition of slavery eventually came on March 25, 1807. Pitt had died the year before, at 46, exhausted by surviving nearly 19 years as prime minister, the latter part spent at war with Napoleon. Newton died just nine months after the act was accepted by parliament, at 82.

Gruffudd was not alone in being fired up by the script. Sewell almost missed out because he was intent on playing Clarkson, and Apted initially wanted him to be one of the villains. “I said, ‘In that case, I’m not interested,’” Sewell recalls. “It was an intensely frosty moment. I was only interested in Clarkson, because of the stand he took.” Sewell returned for a separate audition and was hired. “It is the first time I’ve managed to change anyone’s mind,” he says. “That’s an indication of how much I wanted it.”

The film has drawn criticism even before its opening — from the office of Ken Livingstone. The mayor of London’s policy adviser Amazing Grace is released on March 23 on equalities, Lee Jasper, has accused Apted of “prettifying” the slave trade. Jasper, speaking as secretary of the National Assembly Against Racism, says: “It seeks to give the impression that one man freed millions of slaves, and negates the contribution of the enslaved Africans to their own freedom.”

It is true that only one black actor, Youssou N’Dour (better known as a musician), appears, as an African slave who buys his freedom, settles in London and writes a book about his experiences. But Apted is having none of it. “The focus of the film is on politics,” he says. “It represents the views of William Wilberforce, because he was a man who really did change the world. I think we’ve done him and the cause proud.”

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