IN WINTRY light, sodden Oxfordshire fields flash by the train window, dampening Bagehot’s spirits. Seated on the cramped and stained seat opposite, David Cameron meanwhile exudes his characteristic chipperness. Beaming at the lady who offers him coffee, nodding languidly at the red briefcase containing the afternoon’s business, the Conservative prime minister’s calm and officer-like good humour is evident in everything he does. It is his greatest strength as a leader. In all the mini-crises that have punctuated his premiership, only the near-miss of Scotland’s independence referendum is said to have unnerved him much.