Have a look at my Distance-O-Meter:
http://toolserver.org/~ti/distance-o-meter/?bbox=-0.024242,52.045568,-0.0209218,52.0482352&zoom=14&lat=52.0469&lon=-0.02258&layers=BTFFFFFF&radius=300

It draws yellow circles around post boxes and red circles around post offices, so it can help finding non-mapped amenities! :)