 In 1961 , the Portland Spy Ring was uncovered . Peter and Helen Kroger were found to have been involved , while living in Ruislip . They were visited each Saturday evening by Gordon Lonsdale and were eventually placed under police surveillance . The Krogers were eventually arrested and found to have codes , a microdot reader and film of the Admiralty Underwater Weapons Establishment in Portland Harbour concealed within ordinary household items . A radio transmitter hidden in the garden was not retrieved until 1977 . 
