If the water is going high in the WC pan when you flush the cistern water into it there will be a blockage or partial blockage of some sort. My toilets do this every couple of months and I always find the outlet of the inspection chamber - where all the drains come together to run out to the main drain in the street - is partially blocked.
The water in the pan rises quite high and then very slowly sinks back to the normal level.
With experience it now takes as little as 10 minutes to clear the blockage - it's always in the outlet of the pit and never in the WC pipes themselves.
I use a 2M length of stiff plastic water pipe (barrier pipe) to clear the accumulation of paper waste back into the pit - After a little while everything suddenly drains away and I clean up with a hose pipe.
I hope your problem is as simple as the one I often get - it's an easy fix.
Good luck...
Peccavi, June 2009
