It's a low pressure seal - between the cistern and the pan.
A squidgy washer called a Donut is pressed against by a metal plate - a wing nut each side at the back applies the pressure - easy to tighten and that might be a cure.
If you squint along the gap between top and bottom it should be parallel but if the cistern is tilted then a good seal will be difficult to achieve.
Worst-case you may need a new donut washer - a relatively easy job.
Turn off water supply and flush the cistern.
Undo the wing nuts underneath at the back.
Disconnect the water inlet pipe.
Remove the two screws holding the cistern to the wall
Lift the cistern up and away and pour any remaining water into the pan.
Remove the old donut washer and fit a new one.
Put it all back together again - check the level by squinting along from the side and do not over-tighten the screws that hold the cistern to the wall.
20 to 30 minutes - a screwdriver and a spanner.
Good Luck..., June 2016
