If anyone’s life is worthy of a book, it’s that of Charles Ignatius Sancho. The acclaimed writer and composer was born on a slave ship crossing the Atlantic in 1729, where both his parents died; arrived in London, where he suffered hardship and danger; found love; met the king; and became the first black person to vote in Britain.

Now, a moving retelling of Sancho’s life by the actor Paterson Joseph has won the Royal Society of Literature’s prestigious Christopher Bland prize – awarded to a debut writer who has published their first book over the age of 50.

The Secret Diaries of Charles Ignatius Sancho, a novel written in the form of a memoir, revives the intricacies of Sancho’s journey and of Georgian London where he lived. Joseph, who has been enthralled by Sancho’s story for almost 25 years, said he was on a mission to rescue a great man from historical oblivion.

“The first thing that made Sancho interesting to me was the Thomas Gainsborough portrait of him,” Joseph said. “The image is of a black man dressed like a high-class gentleman of the 18th century. Who was he? How could he? The incongruity of him made me want to know more. And then I discovered his story, which is extraordinary. He’s somebody you have to know about.

Sancho, who died aged 51, was sold into slavery in the Spanish colony of New Granada and later taken to Britain as a two-year-old orphan and gifted to three Greenwich sisters, where he remained for 18 years. Unable to bear being a servant to them, he ran away to the Montagu House in Blackheath, where John Montagu, 2nd Duke of Montagu, is said to have encouraged his literary education.

After spending some time as a butler in the household, Sancho left and started his own business as a shopkeeper. He also published essays, plays and books and became involved in the British abolitionist movement, which sought to outlaw the slave trade and the institution of slavery.

As a male property owner, Sancho was legally qualified to vote and became the first known British African to have voted in Britain. By the late 1770s, he was considered to be a man of refinement and an acute observer of English life.

“There was just a dearth of knowledge in my circle of black Britain before 1948,” Joseph said. “I had presumed stories about the presence of black people in England before the mid-20th century were mythological.” It was only through reading books such as Gretchen Gerzina’s Black England that he found out about “many figures from Roman Britain through to Sancho and beyond”.

“That made me think this is a missing part of my history, and a missing part of my sense of belonging. Because when you’re not included in the story of your nation, then you feel that you are a stranger. Even though this is your only country, the only one you know, the only language you speak.”

The actor, who has starred in shows including Vigil, Noughts + Crosses and Peep Show, originally wrote the story as a one-man stage show that he performed in 2018. He said it was “ridiculous and a bit criminal” that stories such as this were not more widely known.

“The 18th century is the least studied century in history. It’s unfathomable to me, because it’s the foundational period of our national history. We were at war with Scotland, America left the mother country, France was chopping off the heads of its monarchs, and we were at the height of our trading in African captives, the foundation of our economy.

“The more we know about our country the more peaceful it will be. It’s ignorance that leads a lot towards antagonism and misunderstanding across the board.”

Awarding the £10,000 prize, the chair of this year’s judges, Lemn Sissay, said Joseph was “clearly a writer in an actor’s body. Many thespians feel the urge to inhabit the world of a writer, but few can fulfil it to the degree of Paterson Joseph. He inhabits characters and scenes as Dickens does, through the character and story.

“The Secret Diaries leaves the reader frowning at the audacity of history for leaving out such a brilliant character. Equally, we are slightly in awe of the author for turning history around.”