By day they were ordinary civilians — from dentists and clergymen to gamekeepers and roadmenders – in a Britain gripped by fear of imminent invasion by Hitler’s blitzkreig troops. The only clue to their alter egos might have been the pieces of paper in their pockets – informing any police officer suspicious of their behaviour “to ask no questions of the bearer but phone this number”. But new details have now emerged of the highly secretive role played by a “resistance” army of fit young men and women chosen as would-be saboteurs and spies in the event of a German...