The interior sets for the two houses are the reverse of the real life exteriors. The sets show the stairs and landing window to the left of the front door for each house but the exterior shots show the landing windows to the right.
The voiceover explaining how great men are undone by trivial things says King John died from a surfeit of Lampreys; traditionally this was said about King Henry I.
It would not have been possible to put a tape recorder and the bomb inside the radio - it wasn't big enough to accommodate them. Cassette tapes weren't out until seven years after this film was made and so reel-to-reel tape recording was the only method available, and the reel-to-reel tape recorder is much bigger than the radio. The radio had also to accommodate the bomb, which is quite sizable. However, please don't let this spoil your enjoyment of a very good film.
When the radio is "spinning" through the air, the wires holding it at each corner are visible.
Shortly after arriving at The Green Man, Sir Gregory and Jo are in the bar and Sir Gregory, who wishes to be incognito, tries to think of a name by which he will sign the register, but when he arrived he told the landlord that he had booked a room. If he now choose a different name, then the landlord will say that he doesn't have a booking in that name.