When Vasiliy Mitrokhin walked into the British Embassy in Riga (the Latvian capital) they couldn’t believe their luck.
Fortunately someone at the Brit Embassy took him at face value and made an appointment for him to meet someone in the intelligence service.
Mitrokhin was one of the few people to have access to the entire KGB archive and had papers for all past and current missions. Over a long period he had copied their papers and hidden his copies under his house. He knew that one day they would be his passport out of the Soviet Union.
He brought them all out with him when MI6 assisted his passage from Europe into the UK. There is an excellent book you can buy by Christopher Andrew ( who interviewed him).
The Russian scientists could never work out where and how they were getting accurate instructions on how to build a Nuclear Bomb. They had been helped by one Senior Airforce Officer in the USA who had walked into a Russian embassy in America and handed over a complete file of how to build a bomb.
Below is an online archive the Americans have set up but you can also access Vasiliy Mitrokhin material via Churchill College, Cambridge (open to the public).