John R. Dilworth was singing during the 'Bop Em Sock Em Robot Toys' fight between Muriel and Eustace.
It is revealed that Muriel has an Aunt Gertrude, when Shirley inadvertently contacts her instead of Horst. We only hear her voice however, and she has a strong Scottish accent. When Shirley contacts Gertrude, she tells Muriel to start using vinegar in her cooking, and this becomes a running gag throughout the rest of the show.