Underkill — Bruce Crowther

The assassin came to London, confident he would follow his instructions to the letter. As political leaders of East and West gathered for a Summit Meeting, security men began a race against time to find the man before it was too late. The lives of many people would be affected, including a Cabinet Minister and his wife and her lover - and another faceless man who had lived half a lifetime in alien England waiting until his masters were ready to make their move.