Elodie Frenck plays a dual role in this episode: Her usual dumb blonde secretary Marlène (a stereotyped Marilyn Monroe character) and the exact opposite, her sister Solange, an old virgin who runs a boarding house.
Lawrence mentions Inspector Japp, from Scotland Yard. That is a recurring character in Agatha Christie's novels. He is a good friend of Poirot (and, literally speaking, he plays the part of Lestrade for Sherlock Holmes).