The cardboard backgrounds are a reference to the "Paddington Bear" (1976) animations. Director Paul King later directed the live action adaptation of Paddington (2014).
Shot in 5 weeks.
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.