The Domino Men - Jonathan Barnes

In an earlier century, Queen Victoria made a Faustian bargain, signing London and all its souls away to a nefarious, inhuman entity. Now, generations later, the bill has finally come due. . . .

An amiable, unambitious London file clerk, Henry Lamb leads an unremarkable life--until the day he learns he's expected to assume the covert responsibilities of his universally despised, now comatose grandfather. London is at war, and a shadowy organization known (to a very few) as the Directorate wishes to recruit Henry to the cause. All he has to do is find "the girl," save the world from the monster Leviathan, and defeat the unspeakable evil lurking in the cellar of 10 Downing Street: the serial-slaying schoolboy twins known as the Domino Men.