The espionage elements added to the film are inspired by the fact that the author of the book, Arthur Ransome, had actually worked for British Intelligence, spying on the Russians. The code name "S76" that appears in the film was Arthur Ransome's actual code name.
The opening journey was filmed on the Keighley and Worth Valley railway. The same line used in The Railway Children.
The book's character, Titty, was based on a real girl who used it as a nickname. She got the nickname from the poem about "Titty and Tatty Mouse". The filmmakers decided to change the name to the other mouse.
Dan Travers, who was the diving and marine supervisor on the film, is the son of Virginia McKenna, who played Mrs Walker in the 1974 film of Swallows and Amazons.
Many of the young cast had not acted professionally before. For Dane Hughes (John Walker), Bobby McCulloch (Roger Walker), Seren Hawkes (Nancy Blackett) and Hannah Jayne Thorp (Peggy Blackett) this was their first screen outing.
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