if you take the top off the toilet and there is water in it then it will be the big round or square plastic bit in the middle you will have to remove this to get to the plastic film inside this can ripp or just get woren and that is why the water will not flush you will have to turn the water off to the toilet befour doing this job but there will be a turnoff to the toilet on the pipe next to it good luck : )
J,R
December 2005