Where the Marble Arch stands today in London was once the Tyburn gallows — or The Hanging Tree. The walk toward those gallows, along Oxford Street and past the Mayfair mansions, has a bloody and haunted history as the last trip of the condemned. Some things never change. Blood and ghosts have returned to those mansions of the super-rich. Peter Grant’s been put on the job, as England’s last wizard and the Metropolitan Police’s reluctant investigator of all things supernatural…