MI5's dramatic interception of secret signals to Moscow from a hidden base in Wimbledon uncovered the true extent of Soviet espionage in Britain. Intelligence expert Nigel West reveals how MASK, the codename for one of the most secretive sources ever run by British intelligence, enabled Stanley Baldwin and his cabinet to monitor the activities of the Communist Party of Great Britain and track wireless traffic between the Soviet Union and its Comintern representatives abroad, in countries as far apart as the United States, China and Austria. The Government Code an