Susanna Clarke is a member of the elite group of authors who don’t write enough. In 2004, the bestselling debut from a cookery book editor seemed to promise an unfailing fountain of the creative imagination: Jonathan Strange and Mr Norrell, a three-volume reworking of Britain’s military tussle with Napoleon, but with added fairies, felt like Jane Austen brewed up with spells and a dash of the Brontës’ Angria sagas.