Over the four Twitten books, I drew many bits and pieces from the radio series, but tried always to use them differently. The borrowing I’m happiest with is the attractive villainess Adelaide Vine, played by the gorgeous Janet Ellis as a regular rival to Mrs Groynes. By making her the lone survivor of a gang of con artists wiped out by Mrs G in The Man That Got Away, I gave her a strong motive for revenge against our favourite charlady-cum-criminal-boss, and she returns here, in Psycho by the Sea.

As for the department store setting, again it was researching newspapers that drove my interest. Those shops stocked everything, from dog meat to fur coats, and of course the system of suction tubes for carrying cannisters to the cashiers was too good not to use.

“The cash has vanished, so where do you think it’s gone?” Mrs G asks her gang, theatrically.

And when they look blank: “It’s up the bleeding tubes!”