A first screenplay was written by Peter Ustinov, who was also the first choice for Dennis Price's role as Dunstan Dorcester. He may have contributed to the script in collaboration with the credited Patricia Moyes, who had, at one time, been his secretary.
The used car salesmen Dunstan and Dudley (Dennis Price and Peter Jones) were based on characters from the BBC radio comedy series "In All Directions" broadcast during the 1950s. The radio characters were known as Morry and Dud and were played by Peter Ustinov and Peter Jones who also wrote the scripts together with scriptwriters Frank Muir and Denis Norden. Their catch phrase "run for it!" was reprised in the film.
The opening scene at the railway station was shot at Hertford East station.
The burbling sound made by Palfrey's car is the same sound effect used for Sidney Stratton's experiments in "The Man in the White Suit" (1951).
The 'Bellini' sports car driven by Terry-Thomas in the film, is in reality the actual Aston Martin DB3S which was once driven by Sterling Moss in the Mila Migilia and has been valued recently at £7 million.
Stirling Moss never drove a DB3S in the Mille Miglia and no DB3S is worth £7 million.