The Lamb and Flag is almost certainly the oldest pub in Covent Garden, with a history dating back more than 300 years (records date it to 1623), and its hard to find a more characterful, local, pub in this part of London, deep as we are in touristland.
The downstairs bar, with front windows opening onto the street/smoking courtyard is nice enough, but if you're looking for a seat you're better off entering through the small door down the side alley and heading up to the cosy upstairs room, which is a little quieter and even has live jazz every Sunday evening from 7.30pm. Last time your author went, even though he felt jaded after a weekend of excess, the atmosphere was absolutely fantastic.
It comes thoroughly recommended, as does the Sussex Best, which is usually on tap. For a proper review from a magazine, see http://www.timeout.com/london/bars/reviews/12863.html