Richard Attenborough was originally considered for the role of Harry, by director Alexander Mackendrick.

Tommy Cooper was suggested for the role of One-Round but stage commitments prevented him from auditioning.

William Rose claimed to have dreamed the entire story from start to finish.

According to both Alexander Mackendrick and Alec Guinness, Mrs. Wilberforce's birds in the film were voiced by Peter Sellers.

Premiere voted this movie as one of "The 50 Greatest Comedies Of All Time" in 2006.

The Packard motor car used by the thieves as a getaway vehicle belonged to Michael Balcon.

The piece of music that the gang pretends to rehearse is the Minuet from Luigi Boccherini's "Quintet in E major, Opus 11, Number 5". Each time one of the gang members is killed, the arrangement of the music changes from quintet to quartet to trio etc, as if it is being played only by the surviving members of the gang.

The producers originally rejected director Alexander Mackendrick's choice of Katie Johnson for the role of Mrs. Wilberforce on the grounds that she might be too frail for the project, and so they cast a younger actress - who died before filming began.

The robbery at the beginning of the film took place in Cheney Street near its junction with Battle Bridge Road, immediately to the west of Kings Cross station.

The tunnel mouth is the south end of Copenhagen Tunnel, the second tunnel on the main line out of King's Cross station. There are real houses in the area, but the rear exterior of Mrs. Wilberforce's house is a set built in front of a blank wall. Exterior scenes at the front of the house were shot on Argyle Street, about 3/4 mile south (see goofs entry); the interiors are a studio set.

When the parrot flies out of the living room and into the hallway, it clearly says "Alec Guinness" as it lands.