The portrait of Reuben Astwell that conceals a safe is in the style of Tamara de Lempicka, one the most fashionable portrait painters of her generation and a leading representative of the Art Deco style.
The magazine read by Lady Astwell, "The Queen", was originally a society magazine first published in 1861.
The building used for the chemical factory in this story was the Boots The Chemists D10 Building on the Beeston, Nottingham, England site.
For the shot of the train arriving at a London station, a short clip has been lifted out of "The Plymouth Express" (series 3, episode 4). It's a different locomotive from the one which set out, and the naval officer walking down the platform has a small rôle in the earlier episode.