Theres a big round squishy washer that forms the seal - a Donut washer - compressed by two wing nut bolts that pull on a metal plate. You can sometimes cure a leak here just by tightening the wingnuts - low down at the back - one each side.
Failing that it's a relatively simple job to lift the cistern off and fit a new washer - allow perhaps an hour the first time you do it.
Isolate water - flush loo - disconnect water inlet - remove top lid - undo wingnuts - remove fixing screws at back of cistern - lift cistern up and tip remaining water into the pan.
Squint sideways along the line of the join - you should see the donut washer as a thin black line - it should be parallel - it it's not then an effective seal will be very difficult.
For a picture use Google - select Images - search donut washer
Good luck...
Peccavi, April 2010
