The spellbinding stories of Mary Poppins, the quintessentially English and utterly magical nanny, have been loved by generations. She flew into the lives of the unsuspecting Banks family in a children’s book that was instantly hailed as a classic, then became a household name when Julie Andrews stepped into the title role in Walt Disney’s hugely successful and equally classic film. But the Mary Poppins in the stories was not the cheery film character....