One of the strangest encounters of my life took place in the basement of an anonymous office block just behind Buckingham Palace in London. There was a big state dinner that evening and the area was stiff with security. Did any of the police notice two well-known terror leaders slip through a doorway? Gerry Adams and Martin McGuinness, both key figures in the Irish republican moment, had been bidden to the meeting by a former MI6 officer. Let’s...