Short answers, pulled from the story.
MI6, formally the Secret Intelligence Service, was officially acknowledged to exist in 1994. That year the Intelligence Services Act 1994 was introduced to Parliament, placing SIS on a statutory footing for the first time and providing the legal basis for its operations.
The title C derives from the first director of the service, Captain Sir Mansfield George Smith-Cumming, who signed all correspondence with his initial C in green ink. Every subsequent chief of SIS has maintained the same practice when signing documents to preserve anonymity.
The Zinoviev letter was a forged document, purportedly from Grigory Zinoviev of the Communist International, which MI6 leaked to the Daily Mail in October 1924. Published on the 25th of October 1924, it contributed to the defeat of Ramsay MacDonald's Labour government in the election of the 29th of October 1924. Whether MI6 knew the letter was a forgery at the time has never been definitively established.
Kim Philby, known by the cryptonym Stanley, was a Soviet agent who rose to a senior position inside SIS. From his post in MI6's anti-Soviet Section IX from 1944, he passed all British intelligence on Soviet operations to the USSR, including material shared by the American Office of Strategic Services. He also sabotaged the defection of Soviet officer Konstantin Volkov in 1945, who had offered to name Soviet agents inside British intelligence.
Oleg Penkovsky was a GRU colonel recruited jointly by MI6 and the CIA. He provided several thousand photographed Soviet documents, including Red Army rocketry manuals that allowed US analysts to identify the deployment pattern of Soviet SS-4 and SS-5 missiles in Cuba in October 1962.
A joint British-Polish study published in July 2005 found that 48 percent of all intelligence reports received by British secret services from continental Europe during 1939-45 originated from Polish sources. The liaison between the two services was facilitated by SIS officer Wilfred Dunderdale.