Walrus: Click here for information about George Formby at IMDb (The Internet Movie Database). I see it doesn't list a song with "Maginot" in the title—which isn't too surprising. IMDb's data is impressively detailed about recent films, but it's often incomplete when you're searching for information about the soundtrack of old movies.

However, I see GF was in a film called "Turned Out Nice Again" (1941). Since that phrase occurs repeatedly in the song you quoted, I'm guessing that's the film you had in mind.