All wall sockets have an earthing point on their rear, so you'd need to unscrew it from the wall, see if it's the pathetic small screw terminal, or the integral screw recess built into the rear assmbly (better makes like MK use this). If this sounds overly complicated, don't worry, it will be readily apparent when you look at the rear. The anchor point for the yellow/green wire is what you're after! DO REMEMBER TO TURN OFF ALL YOUR MAINS POWER AT THE SWITCHBOARD FIRST THOUGH !!!

Cheers

George