When Northern Rock first ran into trouble in the autumn of 2007, worried customers queued outside branches from the early hours in an attempt to get their money out. ‘This is the first run on a British bank since Mary Poppins,’ said someone. It was one of those easy jokes which succinctly sum up what is going on. In the film Mary Poppins the run on the bank is a mistaken one, triggered by a child loudly demanding the return of the money which a benign father has banked for him. The idea that depositors might lose their money in Northern Rock was equally mistaken.