Leaks on close coupled cisterns usually come from the donut washer that sits between the two units - Cistern & Pan.
You may see two wing nuts at the back of the pan - one each side pointing to the floor. Sometimes (quite often) you can fix a leak by tightening these wing nuts - they compress a metal plate onto the squidgy donut washer and form the seal...
http://www.diydoctor.org.uk/project_images/toiletleaks/cckit.gif
Good luck...
Peccavi
December 2009