By 1965, black and white filmmaking was very much on the decline in the UK. This was one of the last feature films made at Pinewood Studios in black & white.
Most of the production team had worked on the previous year's 'Carry on Spying' (1964) where the producers had decided that would be the last Carry On film shot on black & white film. Originally, only the beginning of "The Big Job" (the original robbery was set in 1950) was planned to be in black and white and the modern scenes shot in color but when carrying out a pre-production audit of what props and cameras would be available for this film's shoot, it was noted that there was still a lot of unused cans of perfectly good 35mm monochrome film stock available. As there were no major black & white 35mm productions planned for the major UK studios in the near future and as television productions tended to use 16mm film or videotape, the production company was offered the last of the old unused monochrome film stock on the cheap. They duly obliged, which is why this film became a completely black and white production at the last minute and why it was one of the last black and white films shot at Pinewood.
The shooting script was titled 'The Great Brain Robbery' after the leader of the gang, George Brain (Sidney James).
At the end of the film, Joan Sims as new head of the gang, plans to rob a train from Edinburgh with used bank notes worth five million pounds. This film was made two years after the Great Train Robbery; however, that train was from Glasgow and was carrying 2.6 million pounds in used bank notes. Joan also mentions changing the signal to red which occurred during the Great Train Robbery. Also Sidney James' character's name is Brain, hence the Great Brain Robbery. These are all obvious references to the Great Train Robbery.
This film was intended to be a colour production. However the film stock supplier found they had a surplus of monochrome negative and wanted to get rid of it as black and white film making in the UK was on the decline by the mid 1960s and some of the stock was short dated. They cut a deal with the producers of this film, letting them have it for half price, and thus saving a considerable amount on the production budget.