I would have thought this had been asked before, but I didn't see anything in the FAQ or find anything searching.
What did the work consist of? I imagine the real agents, no matter how elite, spent most days doing paperwork, as even the Fleming novels acknowledge. Did they really have licenses to kill and if so, have the records of targets been declassified? Was there an equivalent to the Q Branch?
What were the real objectives of their missions? The only one I've heard of is the overthrow of Mohammad Mosaddegh.
This specific period was a complete joke for MI6. This was not a well funded organization in the post war years, due in part to economic crisis that led to govt. budget cuts.
(Urbach, Karina, and Jonathan Haslam. Secret Intelligence in the European States System, 1918-1989. Stanford, California: Stanford University Press, 2014. eBook Collection)
There were not many mission to attack or destabilize the Soviets because of this and the fact that the measures didn't seem very cost effective, or worth the kind of follow up investment required without allies. In your exact proposed timescale we have Operation Stopwatch, 1953, a joint U.S. U.K. operat to tap Soviet phone lines with a secret tunnel in Vienna. There was one small problem; the stenographer for the planning meeting was a Communist spy. He handed over the plans so the op was compromised before it even started.
( Dorril, Stephen. "Intermarium." In MI6: Inside the Covert World of Her Majesty's Secret Service. New York, New York: Touchstone, 2000.)
Stopwatch was a plan to tap Soviet phone lines in 1953, that was compromised before it even started. The most telling anecdote of what British Intel was like in the 50's is that the head of the anti Soviet counter intel department was a soviet spy who made a clean getaway. Harold 'Kim' Philby was the man. He was recruited in the 30's for WWII. It's not clear just how much information he passed along to the Soviets.
[Trevor-Roper, Hugh, and Edward Harrison. 2014. Secret World, The : Behind the Curtain of British Intelligence in World War II and the Cold War. London: I.B.Tauris, 2014. eBook Collection (EBSCOhost), EBSCOhost (accessed April 20, 2015).]
And Philby fled to Russia in 1963, but this was after he retired from intel. he was suspected by MI5 (home intelligence) but nothing was confirmed until he bolted.
There was however some basis for M. One was Rear Admiral John Godfrey, the man whom Flemming worked under at Naval Intelligence. He thought he was M at any rate: "He turned me into that unsavoury character, M."
(Macintyre, Ben (5 April 2008). "Bond – the real Bond". The Times. p. 36.)
Also the head of MI5 during the war Maxwell Knight may have inspired the character. He is supposed to have signed his dept. memo's M. same reference.
I hope this helped.
Your terminology is confused. You either mean an MI6 officer, or an agent. Since agent details are rarely, if ever, revealed, I'm assuming you mean MI6 officers.