The cardboard backgrounds are a reference to the "Paddington Bear" (1976) animations. Director Paul King later directed the live action adaptation of Paddington (2014).
The character of Bunny was based on Simon Farnaby's grandfather. He mentions in the commentary that his grandfather once had a secret hole in a his house that tunnelled to the pub next door.
Though time is often seen passing in the scenes in Edward's flat, the yellow lighting is intentionally kept the same throughout.
Shot in 5 weeks.