Espionage: A Free Agent (Michael Powell, 1964, TV) torrent

Leo Marks, who wrote Peeping Tom, also wrote this story. Anything with Powell and Marks is worth checking out, it's as if they wanted to recreate the old Powell-Pressburger magic.

Marks had been a spook himself, a cryptographer, so he must have had some knowledge of "the game". This one is as much melodrama as thriller, as two enemy spies fall in love with each other; you can imagine Garbo playing this sort of thing in the 1930s.