Interestingly, the criminal Mr. Big in this episode was played by George Baker, who later achieved fame on British television as Ruth Rendell's Inspector Wexford.
Interestingly, John Gregson faces here exactly the same puzzle he was faced with when he was a police detective in The Lavender Hill Mob (1951), namely to discover how gold-robbers intend to smuggle the gold out of the country.
This episode has 50 minutes without commercial time.