The piano music being heard under the titles, played on the piano, and on the radio is "Für Elise." Elise was Sophie's code name when she went to France as a British intelligence agent. There's another irony as "Für Elise" was written by Beethoven in 1810 but not published until 1867, forty years after his death. Elise was a pseudonym for the composer's "Immortal Beloved," whose real identity is often speculated on.
The twin-engine biplane featured is a DeHaviland Dragon Rapide, a popular and robust aircraft of which many, despite the type's plywood construction, are still flying as of release.
Hawtrey mentions a radio operator's "fist." This was a real term used during the War by the clandestine services to mean a particular wireless operator's personal style while sending Morse code. To someone who was very experienced, an operator's fist was as unique as a fingerprint or a signature, and if an experienced operator had trained someone, he would instantly recognise their fist - or that the fist of an operator he was listening to was not that of the person he had trained. If he had stepped forward to say so, the SOE would have done well to listen to him.
The last ever episode of "Foyle's War."
The location for the external shots of the MI6 HQ was the former Cunard building on Canada Boulevard at the Liverpool waterfront.