"Put her into Brixton jail with all the other 18Bs", says an annoyed lady diner on seeing Sally. This references Regulation 18B of the Defence (General) Regulations 1939, which allowed for those suspected of being Nazi sympathizers to be interned; some indeed being housed in H.M.P. Brixton.
When Jan Orlock (Albert Lieven) explains the German plot to Sally Maitland (Anna Neagle), he says that the plot was inspired by a World War I-era explosion in the Port of Halifax. In fact, he pretty accurately describes the events that took place on the morning of December 6, 1917. Two vessels - the Norwegian S.S. Imo and the French S.S. Mont-Blanc - collided in the port at very low speed (about one knot). The Mont-Blanc was loaded with explosives, benzol (a highly flammable fuel) and other volatile cargo. After the two vessels struck each other, drums of benzol broke loose and flooded the deck. As the two ships disengaged from each other, sparks ignited the fuel, started a fire and caused an explosion approximately twenty minutes after the initial collision. The explosion killed nearly two thousand people, levelled all structures within a half-mile and caused a local tsunami.
This movie appears to amalgamate various real events of September 1940. "The Evening News" headlines a time bomb explosion outside Buckingham Palace. The Palace was bombed seven times during the war (including an incident with a fifty kilogram delayed action high explosive, which harmlessly landed in the grounds), most notably on September 13, when a bomb landed in front of the Palace, damaging Queen Victoria's memorial and razing the chapel. However, King George VI and Queen Elizabeth were in residence during this morning attack, unlike in this movie.
The scene of the morning following the boarding of the ship by the crew of the raider, Mrs. Towcester - played by Margaret Rutherford - states that "From now on I am going to live! I am moving to Balham.". Margaret Rutherford was born in Balham.
The aircraft (shown in stock footage) which are to bomb the explosive-laden ship are Lockheed Hudsons, then a mainstay of RCAF and RAF coastal operations.