Hell's Underground: The Demon Assassin: Book 2 by Alan Gibbons
Having once risked everything precious to him to halt King Lud's progress from the ancient bowels of London to the city's modern streets, teenage Paul Rector knows his task is far from over. Paul also knows that, like his brother John and his great uncle Harry, he is both demon and man. Will it be fate that decides which side wins out? To find out, Paul takes Hell's Underground back to London of the Blitz in World War II and becomes involved in defeating Harry Rector's plot to assassinate Prime Minister Churchill. His main weapons are fire and fear. Along the way, he learns more about the Rector curse and once again endangers not only his own life, but that of the people he comes to regard as a second family.