On a close-coupled system where the cistern sits directly on the back of the pan a leak sometimes happens from the joint between the two.
A large squishy washer - doughnut washer - makes the seal - it gets pressed down by a metal plate which you can tighten up with two wing nuts at the very back - pointing downwards.
Check this first - it costs nothing....
Also squint sideways along the small gap between the two - the gap should be parallel - if the cistern is sitting on the pan tilting forwards or backwards then that may well be the cause of the leak.
Good luck...
Use Google - select images - enter donut washer
Peccavi
January 2013
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