Yank title Mr. Potts Goes to Moscow
I was sure this was the film I'd seen before where a sanitary engineer, with plans for a revolutionary new type of toilet cistern, gets mistaken for a nuclear scientist. Identical briefcases (with plans) get mixed up, and by a roundabout way the sanitary man defects to Russia, or so all concerned (British and Russians) think he wants to develop his system behind the Iron Curtain, because he is not appreciated in the UK.
Phew - so it wasn't that film, and I only stuck with this one because of the superb performance of George Cole as the mistaken engineer. Not a barrel of laughs, but George was so good, it was worth it just for him - vague innocent put upon, not understanding what was going on, which he was so superb at.
Now if only I could think of that other film - anyone?