In the summer of 1939, most of Britain was preparing for war. But down in the Suffolk countryside, a gentler clash of civilisations was taking place. The Sutton Hoo excavations were one of this country's most important archaeological discoveries: 'Britain's Tutankhamun' screamed the headlines of the day. In ''The Dig'' John Preston has used these events, and the stories of actual people involved, to enact a peculiarly English drama: gentle yet powerful.