The rise and fall of the Mitford sisters is like one of those earthquakes we’re due on a regular rotation: eight years out from Gucci’s much-documented Never Marry a Mitford jumper, four years after the BBC drama The Pursuit of Love, a new TV show appears fortuitously to bring them back into the public consciousness again. Here they come, out of the mists of time, the seven children of a minor member of the House of Lords: Nancy, of course, the author of Love in a Cold Climate and The Pursuit of Love and probably the most famous in her...