He began life on a slave ship but by the time of his death had become a highly popular composer and best-selling writer whose published letters created a sensation. In his day he was a notable London figure painted by Gainsborough and close friend of the actor David Garrick and writer Laurence Sterne.

Yet his name is not in the history books - a situation that the National Portrait Gallery is currently trying to retrieve with its biographical exhibition Ignatius Sancho: An African Man of Letters. This is a small but significant exhibition. For students of black history or 18th-century society, it will provide illuminating insight into a remarkable man’s life and times.

Ignatius Sancho was born in 1729 on board a slave ship en route from West Africa to the Spanish West Indies. At the age of two he was brought to England and given as a “gift” to three sisters who lived in Greenwich. Contemporary pictures suggest he would have been dressed in exotic clothes and treated as a “toy”. Small wonder that at the age of 20 he ran away and sought protection with the Montagu family, as a butler in their comfortable Thames-side home.

At this point there were about 20,000 black people living in England, many in conditions of servitude or slavery. Sancho, however, rose to become a member of the small black elite. His letters reveal that as well as composing music for the harpsichord, he had a passion for the theatre and was friends with numerous artists and authors.

At one point a legacy from the Montagu family gave him the chance of giving up domestic service and fulfilling his dream of becoming an actor. The gamble failed. The money seems to have frittered away on wine, women and song (“My last shilling went to Drury Lane”) and Sancho married and went back into service. Presumably a sadder and wiser man, towards the end of his life he went into business as a grocer.

This in essence is the story of Sancho’s life, which the exhibition presents through portraits, engravings, artefacts, and documentary evidence. The centrepiece of the display is a harpsichord playing Sancho’s musical compositions - ironically juxtaposed with the emblems of slavery, the whip, metal collar and shackle that were imposed on slaves. This is the real heart of the exhibition.

A stream of letters from Sancho to Laurence Sterne, the clergyman author of Tristram Shandy, express his horror of slavery. Conscious of his own privileged position he entreats Sterne to take action to address the issue. Later these letters were used by the abolitionists in their campaign against slavery. This puts Sancho in an important position at the forefront of the abolition movement, coming before the better known campaigner Olaudah Equiano.

One of the strengths of the presentation is that Sancho is portrayed warts and all, with all his faults, achievements, joys and sadnesses intact. The ambiguity of his position as a grocer, for instance, is not overlooked. To attain his independence from patronage he ran a shop that sold tobacco and sugar - two of the commodities that underpinned the triangular trade between Africa, the New World and Europe, which included the shipment of slaves to the Americas. A paradox that could easily be used for class discussion.

Nevertheless Sancho’s reclamation from the historical shadows rediscovers a black presence in Britain since the 18th-century.

National Portrait Gallery until May 11. Free teachers’ notes and worksheets are available from the education department, tel Sarah Hartley on 0171 306 0055. The book Ignatius Sancho: An African Man of Letters is also available at Pounds 10.95