The Bagthorpes V. the World by Helen Cresswell
When Mr Bagthorpe gets his latest bank statement, all hell breaks loose! He thinks he's broke and nothing can persuade him that the bank has made a mistake. All he can think about is saving money - and that means that if the Bagthorpes don't raise it themselves, they're not eating it. No shops; no sweets; no arguments. But the Bagthorpes have never really been hunter-gatherers. So, while Granny Bagthorpe retires to her room to scoff a hamper from Harrods, the rest of the family find food - and how to stop it escaping - a whole new challenge!