By a second-storey green-framed window in Baildon's Balti House restaurant at 9 Browgate, a brass plaque commemorates the day in the 18th century when the great Methodist John Wesley preached to a crowd from that very aperture.

Wesley was part of the movement against the slave trade. His hymn, Amazing Grace, is the title of a film made to commemorate this year's bi-centenary of the 1807 Act of abolition passed by Parliament.

Reform always found a voice in Bradford which, during the Civil War, had been firmly on the side of Cromwell's Parliamentary army. Opposition to the slave trade abroad was matched by a swelling antipathy to the conditions of employment in the textile mills of the industrial revolution and support for the Ten Hours' Bill, led by Richard Oastler. His 20-year campaign mirrored the time it took for the bill to abolish slavery to be approved by Parliament.

William Wilberforce, as every school pupil used to know, was John Wesley's younger contemporary who spearheaded the long campaign in Parliament to rid Britain and ultimately the Empire of the traffic in African slaves.

Another William, William Hague, the former leader of the Conservative Party before Michael Howard and David Cameron, has written a 515-page biography of Wilberforce who, judging by two of the book's illustrations, started out life looking like Little Lord Fauntleroy and ended like Voltaire.

Along the way he was traduced by the cartoonist James Gillray. After the Loss of Another Slave Bill is the title of Gillray's 1796 lampoon which shows Wilberforce consoling himself with two large black women. William Hague was himself pilloried during the fruitless years of his often shrewdly directed attack on the financial probity of Tony Blair's Government.

Another illustration shows Wilberforce's handsome London house which was demolished to make way for the present Royal Albert Hall. When Promenaders happily bellow Rule Britannia on the Last Night of the Proms, how many make the connection between the declaration that Britons never will be slaves and the historical location under their feet?

Hague sensibly starts his book with a four-page prologue recounting the triumph of the 1807 Slave Trade Abolition Bill. To have delayed starting with the obvious would have risked causing irritation among readers sufficiently aware that the real story of abolition lies in the Yorkshireman's conversion to the cause as espoused by the Clapham Sect - "one of the most extraordinary and influential coalitions British society had ever seen," says the author.

This group of like-minded evangelicals grouped themselves round the family and hearth of Wilberforce's rich uncle, Henry Thornton, to whose 32-bedroom home in Clapham he added two houses in the gardens to accommodate friends. Hague takes half a page to list the many good works of the Claphamites.

Wilberforce, of course, is the epitome of the man who lost every battle but ultimately won the war. Until the end of his mortal life he declined all titles, honours and ministerial positions.

"Wilberforce often noted that he was ashamed of his luxuries, that he was suffering very little compared to others and compared to Christ, and that he was deeply thankful to be surrounded by his wife, sons and friends who cared for him so well."

On Friday, July 26, 1833, Wilberforce learned that the bill to abolish slavery throughout the British Empire was secure in its passage through the House of Commons. From August 1834, some 800,000 slaves in Britain's colonies would be emancipated, and the warships of the Royal Navy would pursue any suspected slaver.

At 3am on Monday, July 29, 1833, the man who had divided public opinion but ultimately united Parliament, died. Hague acknowledges his seeming contradictions, the accusations of credulity and timidity; but says finally: "In the dark historical landscape of violence, treachery and hate, the life of William Wilberforce stands out as a beacon of light, which the passing of two centuries has scarcely dimmed."

  • William Wilberforce: The Life of the Great Anti-Slave Trade Campaigner, by William Hague, published by HarperCollins at £25.