On the train back to Calais, Tuppence is seen reading "Strong Poison", a mystery novel by Agatha Christie's crime fiction colleague Dorothy L. Sayers.
The picture of Rita Vandemeyer (Alice Krige) in Japanese character makeup is not Madame Butterfly as per the episode plot but Yum-Yum from the performance of The Mikado attended by the characters in "Chariots of Fire" in which Miss Krige played actress Sybil Gordon. (That film took historical liberties as Harold Abrams was not romantically involved with Sybil Gordon. He courted Sybil Evers, a minor member of the D'Oyly Carte company.)
David Walliams appeared in Dr Who The God Complex as Gibbis and Jessica Raine appeared in Dr Who episode Hide as Emma Grayling