 The weather has changed for the worse and it is unlikely that Hitler will ever invade, although that does not stop the group lying down and listening when they start to suspect they have detected a Nazi attempt to tunnel into Britain.Filming took place between 10 August and 25 September 1970, at Shepperton Studios and various locations, notably Chalfont St Giles and Seaford.The film made a number of significant changes, imposed by Columbia Pictures, such as recasting Liz Fraser as Mavis Pike instead of Janet Davies and filming outdoor scenes in Chalfont St Giles rather than Thetford.