My father came across quite a lot of the SOE agents when he was visiting MI5 headquarters during the Second World War. He didn't actually work for MI5, but was a frequent visitor as a GPO telephone engineer. He went there often enough to have his own pass but if he spent all night out on the razzle and didn't have time to go home to get his work clothes including his security pass, he used to show a package of Gold Flake cigarettes. Apparently the MI5 pass looked just like the front of the pack.
I think the film is a much more accurate portrayal of the intelligence services than John Le Carre's books, showing them as bonkers rather than gritty. As I believe Spike Milligan observed when he was a gunner during the Second World War and came across the spooks from time to time, 'military intelligence' is a contradiction in terms.
Plenty a 1985 film directed by Fred Schepisi with a screenplay by David Hare.