Portrait of Ignatius Sancho, 1768, by Thomas Gainsborough

WE all know Paterson Joseph is a fine actor. From TV’s Casualty to Elmina’s Kitchen via the Royal Shakespeare Company, he’s grist to the theatregoer’s mill – thoughtful, strong and serious.

So you would think it comes as little surprise that his 24-year obsession with Charles Ignatius Sancho, born of his reading of Gretchen Gerzina’s Black London and embodied first in his one-man show, Sancho, An Act of Remembrance in 2014, should naturally manifest as this weighty tome. This does not, however, take away from the extraordinary achievement of this debut novel.

Originally from Kilburn, Joseph describes the simple beginnings: “I’ve been at it since 1999. We’ve always had these fragmented bits of information, usually from mad people on Kilburn High Road or outside Brixton tube station talking about Victorian England and Edwardian England. These people knew what they were talking about but it was sort of garbled and half-baked. So Black London, even though it’s not the first and only, was the one that articulated those lives the best and made them come alive.

“And then I was at that point already looking for a character that I could play or give to somebody to write – I didn’t think of myself as a writer, of course. I felt like my training as an actor, my interest in the sort of classical European theatre cannon, lent itself to me being in those shows but, you know, I wanted to find something that I could do where I could be myself and also be black within it. It wasn’t any more complicated than that, almost a vanity project. Then it just took me over and became this mini crusade to get people to know more about black history in the middle of the 20th century in Britain.”

With Obama’s US presidential victory, Joseph struggled to find a UK venue for his one-man show in a “post-racial” Britain, so premiered Act of Remembrance in the US.

“It took until 2018 – would you believe – to find a venue in the UK, a London venue, which was Wilton’s Music Hall. From there I got a lot of interest in the show, people wanted me to do a TV version of it: The first time it seemed to be recognised out of a very small enclave of people who were interested.”

It’s a tremendous book. He brings to the text a Dickensian touch – deftly drawn signature characters, smells, sounds and atmosphere so vivid you’re transported into Sancho’s mind. It reads almost like the script of a very good BBC drama with its old London mix of elegance, squalor and soap-opera suspense.

And Sancho is a rich subject. Born in 1729 in the Atlantic Ocean when it was still slavery’s Middle Passage, he was adopted as a boy by the aristocratic Montagu family in London and was to become the first black man to vote in a British election, a greengrocer, an abolitionist, a musician and composer and an actor when such activity was exceptional for a black man.

The book goes on for a meaty 411 pages in bite-size chunks and perhaps its only fault is its hasty canter through Sancho’s latter years covering those famous episodes at breakneck pace. In the context of this book, though, these landmarks are, in a sense, unimportant as the real pleasure is the depth of imagination Joseph brings to the early years of Sancho’s life, a real feat of literary skill. For a debut novel it’s, frankly, stunning and I’m not surprised it has already won the prestigious Christopher Bland award for a debut novel and been shortlisted for others.

As Joseph reflects: “I didn’t think of myself as a writer but I’ve done this thing and it’s such a labour of love I couldn’t pretend that it didn’t take all of me. I definitely think of myself as being a writer now and I definitely feel that I’ve been through writer’s boot camp!”

The paperback comes out in October, coinciding with the start of Joseph’s tour of his latest play Sancho and Me for One Night Only, a show interspersed with readings from the book.

He laughs mockingly when I ask whether he has reached the end of the Sancho obsession: “Oh, hilarious! There’s a frame of his above my dining room table and it’s a rather big frame and it’s basically Sancho going ‘I’m going nowhere’ so, no. I’ll write other things, but I’ll constantly be touching on his life.”

• The Secret Diaries of Ignatius Sancho. By Paterson Joseph, Dialogue Books, £16.99