Hilary Mantel is one of our most assured and successful novelists. She writes blackly comic novels set in the present and confronts our Tudor past in her Thomas Cromwell novels, Wolf Hall and Bring Up the Bodies. She reimagines famous figures from our history, or imagines for herself the life of a psychic medium in the suburbs of Surrey and Berkshire. In fact, everything she writes is historical fiction, because everyone she writes about must deal with their own past. James Runcie meets a writer who has conjured the ghosts of Henry VIII and Lady Diana, and whose latest collection of short stories contemplates the possibility that Margaret Thatcher was assassinated in 1983.