FT's "Five of the best: dry gin martinis in London"
A very good list of good cocktail bars there really, although I've never been to Duke's.
Rules' upstairs bar is something of a hidden gem now they don't take reservations. Colebrooke Row (and the sister bar at the Zetter Townhouse) are fantastic.
Much better places than the overly-trendy (read just kind of rude) ECC etc.