Modern cisterns often overflow into the WC pan - older ones had the overflow going outside through the wall.
If your cistern leak is into the pan then take the lid off and adjust the little nylon screw inwards towards the valve and do up the locking nut to stop it moving - this is the same as lowering the float into the water to make the valve shut sooner. Sometimes the "rubber" seal in the inlet valve has perished and needs to be replaced but it's usually just an adjustment that's required.
If the leak is a dribble of water from between the close coupled cistern and the pan (an external leak) each time you flush - then it's the "donut" washer that forms the seal between the two - you can often solve this leak by just tightening the two wing nuts that point downwards at the very back and on each side of the pan - they just squish the seal a bit more.
Good luck...
Peccavi, February 2010
