 The text is a reference to the Doctor's actions and dialogue near the end of "The Christmas Invasion".There were also £20 notes featuring producer Phil Collinson.These had the phrase "There's no point being grown up if you can't be a little childish sometimes" printed on them, misquoting the line originally spoken by the Fourth Doctor (Tom Baker), in "Robot", "There's no point in being grown up if you can't be childish sometimes."