A Hard Day's Knight(2011)

Simon R. Green

John Taylor is a P.I. with a special talent for finding lost things in the dark and secret center of London known as the Nightside. He's also the reluctant owner of a very special-and dangerous-weapon. Excalibur, the legendary sword. To find out why he was chosen to wield it, John must consult the Last Defenders of Camelot, a group of knights who dwell in a place that some find more frightening than the Nightside.

London Proper. It's been years since John's been back-and there are good reasons for that.