There's a soft rubbery washer between the WC bowl and the cistern - it's called a donut washer. It's compressed by a flat metal plate pulled down tight by two screws with butterfly nuts - one each side at the back - pointing down.
You can sometimes fix a leak by just tightening up the butterfly nuts.
If you have to remove the cistern - first turn off the water supply - remove the cistern lid - then flush the cistern. Undo the two butterfly nuts and look for two screws holding the cistern against the wall - remove the screws and carefully lift the cistern from the bowl - tip any remaining water back into the bowl. Ten minutes work.
If you do this and ft a new donut washer - then when you put it all back together again have a squint sideways along the gap between cistern and bowl - the gap should be parallel - if it isn't then there will be a leak. And do not over-tighten the two screws into the wall else you might crack the cistern.
Good luck...
Peccavi, March 2013
