The author Graham Greene journeyed to West Africa in 1935, ostensibly to write a travel book. But, claims Tim Butcher, it was a cover for a spy mission on behalf of the British anti-slavery movement which was investigating allegations that Liberia, a state born as a refuge for freed US slaves, was guilty of enslaving its own people.

On a grey January morning in 1935 a steamship of the Elder Dempster line left Liverpool on its standard shuttle service to the west coast of Africa. It was appropriately named — given who was on board — SS David Livingstone, after the missionary, medic, explorer and anti-slavery campaigner whose 19th-century treks through southern and central Africa did much to open up what was then still viewed as the Dark Continent. The ship’s manifest recorded its seven passengers as predominantly the sort one would expect to find on a ship serving British colonial interests. There was a doctor, a shipping agent, an accountant, an engineer and a dutiful wife going out to join her colonial officer husband. There was also a man atypical among camp followers of empire. His address was logged as 9 Woodstock Close, Oxford and his age as 30. His name was recorded as H.G. Greene.

He had been christened with the first name of Henry, but was later to become known worldwide by his middle name. He was the author Graham Greene. Greene always publicly insisted this trip, his first to Africa and his first outside Europe, was a flight of fancy or, as he would later put it in his 1936 account of the adventure, Journey Without Maps, ‘a smash-and-grab raid into the primitive’. There was certainly something fanciful about the presence of the seventh passenger, his first cousin, Barbara Greene.

Graham Greene had tried for some time to find a travelling companion but had failed, largely because his planned itinerary involved an ambitious overland route through Liberia, still one of the darker corners of the continent. But at a family wedding in October ...