Historians always crave that which they cannot acquire. For British constitution watchers, there has always been one special network that never leaks, and of which no note of meetings is taken – the conversations between head of state and head of government. How gloriously fascinating, therefore, was David Cameron’s exchange in New York with its former mayor, Michael Bloomberg, picked up by the directional microphone of a nearby British television crew.Before savouring every line on their inner thoughts on the Scottish Question, it is worth considering what would have happened on Friday 19 September had Scotland voted to separate. First, the Prime Minister would have flown to Aberdeen and on to Balmoral in the morning. You cannot discuss the sundering of her kingdom down the p