I have just had a similar experience and replaced the donut. That did not stop the leak. The problem was the additional rubber washer/seal betwen the siphon and the ceramic (just above the donut). I replaced the siphon which included the seal and now ok.
Keith, March 2009
Ensure its the right way up
OR
tighten the securing bolts
OR
Change the doughnut seal and ensure the cistern is sitting square on the pan.
