Britain’s national debt is on course for £3 trillion this year. Servicing it costs £30 billion a year at 1% interest and £150 billion at 5%, near where long-dated gilts have recently been trading. That’s a number so big it almost becomes meaningless. So, let’s try another way: it’s about the yearly gross pay of nearly four million median full-time workers. That’s the background to Kenneth Rogoff’s warning that Britain has a better than 50:50 chance of a major debt crisis before 2030. Rogoff is not an excitable commentator. He is a former chief economist of the IMF, a Harvard professor and co-author of one of the gravest studies of sovereign debt crises ever written.  [...]