Is it leaking externally or internally ?
External is the doughnut seal between bowl and cistern. if the toilet was positioned and the cistern fitted loose then screwed to the wall then it might be the doughnut washer is not compressing when the nuts are tightened on the plate.
If its water leaking in to the bowl then that's a water level problem where its rising too high and going in to the internal overflow so leaking out in to the bowl so adjust water level.
You could try the kitchen/bathroom sealant top and bottom of the doughnut see if that works.
Its referred to as doughnut because of its shape if its the same I'm referring to.
Hope it helps in some way
Bob UK, January 2014
