Not long after Jeremy Corbyn became leader of Britain’s Labour Party in 2015, his shadow chancellor John McDonnell was at the parliamentary dispatch box debating his opposite number George Osborne when a sharply revealing moment took place.

To Osborne's surprise, and visibly increasing delight, McDonnell pulled a book out of his pocket and began quoting from it. What was it that made the wily Tory's eyes light up? It was Chairman Mao’s Little Red Book.