National Treasures'Geeks triumph over the forces of darkness: nothing could have given me greater pleasure. Combining an exciting story with scrupulous research, Caroline Shenton has done her unlikely heroes proud' Lucy Worsley As Hitler prepared to invade Poland during the sweltering summer of 1939, men and women from across London's museums, galleries and archives formulated ingenious plans to send the nation's highest prized objects to safety. Using stately homes,