Disturbia by Christopher Fowler

After this city hits midnight, everyone becomes equal. That's when anything can happen.
It's 2 am and Vincent Reynolds is running for his life through the stormswept London streets. He's discovered a secret and wants to tell the world - if he can stay alive until dawn.
His adversary - an English gentleman obsessed with puzzles, playing his deadliest game. His allies - a motley crew of insomniacs, misfits and street people. His only hope - to discover the solutions to ten lethal challenges that will lead him, from dusk till daybreak, through the nightlife of a secret city hidden even to its inhabitants.