Joan Leigh Fermor’s biographer turns to Eddy Sackville-West, Desmon Shawe-Taylor and Eardley Knollys and the unusual salon they created at Long Crichel in Dorset, where Nancy Mitford, Benjamin Britten, Laurie Lee, Cyril Connolly, Somerset Maugham, E.M. Forster, Cecil Beaton were among the visitors.