In the scene where they both talk in Joan's house, when Turing reveals that Cairncross was the spy, why does the MI6 agent (Mark Strong) take it lightly? He says something regarding that he placed him in the first place and is sharing secrets with Stalin and not really a clever double agent. What did they mean exactly?
UK and Soviet Russia were allies after The Third Reich attacked the latter. However according to Stewart Menzies (the MI6 agent) Churchill didn't want to share information with the Russians. Because of that they used Cairncross to leak the information they wanted the Russians to receive.
Stewart says Denniston is looking for a spy in their hut and Alan tells them the spy is actually John Cairncross. Stewart admits to knowing this before Cairncross even got to Bletchley; this is exactly why he placed them there – so they could leak whatever they wanted to Stalin since Churchill was too paranoid to share information with the Soviets. Cairncross is unaware that he is being used by them. Stewart says he needs Alan’s help to know what to leak to John and feed to the Soviets.