Halliday eventually concludes that the false nightwatchman has committed 14 safe-breaking jobs across the country, all on the same type of safe, and all completed with genuine keys.

When another safe is broken into, a young man on his way to work sees the thief climbing over the gate, and, realising something is wrong, tries to stop the thieves escaping but is deliberately run over by the getaway car.

Halliday and Ward deduce that Gilson faked his own death after spending years making duplicate keys for all the safes his company produced.

The film is competently photographed, decently acted and cleanly devoid of rough edges, yet as a thriller it is a trifle thin and unimaginative.

The personal asides (Halliday's jocular relations with his assistant and with his neglected but patient wife) fail really to humanise the characters: they remain impeccably reserved stereotypes.

Jack Hawkins gives a conscientious performance as Halliday and makes the most of the script's few touches of humour, and John Stratton plays amiably as the eager Ward.

There are some effective character studies from the supporting cast, notably Meredith Edwards' chatty and inquisitive Welsh garage proprietor and Gillian Webb's comically retiring housewife.