I have just bought a trent wc which does not have a donut seal but a flat thin rubber gasket that says it needs to be glued onto the pan and the cistern. If this is the case and still leaks, how can it be repaired. I have tried doing the seal without glue, but it leaks profusely at the back and looking under the cistern, you can see that the seal is not connecting to the cistern or pan. What should I do? I have checked that the pan and cistern are level.