The Hounds of Vengeance — John Creasey

Z5 is given the task of finding and capturing a Nazi fugitive. It's chief agent, Dr. Palfrey, manages to learn more than anyone expected when he discovers that far from being alone, the man is only one of a growing and powerful band who, resenting their country's defeat, are now bent on revenge upon those that brought it about. He must defeat them and their objective.