When Catherine asks Humphrey what he thinks of the soup she made him, she appears anxious. This is because in an earlier episode "an unhelpful aid", she made the previous detective Richard soup when he had a tropical fever and he said her soup tasted like socks and threw it in a plant pot.
Humphrey while talking with his aunt at hospital references a logic puzzle that states a man from Crete said all Cretans are liars. This is known as an Epimenides paradox, which reveals a problem with self-reference in logic. Epimenides was a Cretan who made the referenced statement circa 600 BC.
Robert Daws (John Green / Marcus Knight) & Wendy Craig (Mary Goodman) also worked together on The Royal (2003) as Dr. Gordon Ormerod & Matron respectively.