“When I’m writing, I know what I want to have next – this scene, that snatch of dialogue, this passage. The theme of predation, the tendency of a society to eat itself, and the Russian-doll structure, evolved organically as I was writing. My books start with four or five stem cells – a mention of the Moriori tribe in Guns, Germs and Steel by Jared Diamond, and the revelation that humanity can regress just as easily as it can go forward. Then we’re whisked off to mid-1970s California in a thriller about a girl reporter investigating a crooked energy company with the help of a man called Sixsmith – the chap to whom the Belgian letters were addressed in 1931.

After a spell in the memoirs of a venal London vanity publisher in the 1990s, on the run from hoodlums and incarcerated it a twilight home, we find ourselves in Korea a century into the future. Here workers are cloned “fabricants”, but one female, called Sonmi-451, acquires intelligence and vision and is condemned to death. Centuries later, she becomes a goddess in a time when the whole of global civilisation has come to an end, and we hear about the final days. At this point, the narrative swivels round, and all the stories are ended in turn, until we’re back in the 19th century, heading for the heart of the slave trade. It’s a head-spinning display of structural and linguistic virtuosity.Robert Frobisher, the musician in the second story, composes a piece of music called Cloud Atlas, in which six instruments overlap with each other “Revolutionary?” he asks himself. “Or gimmicky?” What went through Mitchell’s mind when plotting the book? “I didn’t think about it at all,” he says. Abruptly the story ends and we’re suddenly reading the Isherwood-like letters of a louche bisexual Cambridge music student called Robert who has gone to Belgium to worm his way into a job with a crochety British composer, to become his amanuensis and make love to his wife.

I’d been expecting a sizeable barn as befits such an oceanically ambitious writer, its walls covered in plot blueprints and maps, sheets full of time-scales, arrowhead diagrams, maybe a blackboard with different-coloured chalks. He (and we) learn about the fate of the peaceful Moriori tribe at the hands of the Maoris, and he discovers a stowaway. We begin in the mid-19th century with a journal kept by an American notary on board a ship crossing the Pacific. But this is the smallest shed in the world, an austere little hut with a table and chair, a radio and, mystifyingly, no books.