Obviously from your description it was put in by a retard who didn't have the sense of how to access it for maintenance. Could be close coupled, cistern mounted on pan, in which case it could be the rubber doughnut seal between the pan and cistern is either perished, the cistern bolts are loose, or it's been moved slightly.
The other type is the pipe fed cistern to pan low level as they are called. Either the pipe joint at base of cistern is leaking or the joint at the back of the pan which is a rubber joint. Access; Tiles will have to be removed. But first check their is not an un-grouted area around the tiles forming a cover. I've seen it done a 1/4" marine grade ply board tiles with brass fold flat lift rings fitted to lift the board off magnetic door catches, one in each corner and as board is replaced the magnetic catches snatch it back flat against the wall, very neat.
Nobby
November 2008