I have this problem every few months. You may have a blocked toilet pan but the issue I get is a blockage of the pipe leading from the inspection pit outside the house. Foul water builds up in the pit and back pressure stops the toilet pan from draining quickly.
I take the lid off the pit and use a piece of plumbers barrier pipe about 2M long both stiff and flexible enough to poke the obstruction away. Then a quick hose down.
It's a ten minute job. If it is your pan that's blocked you can get a drain cleaning spring that will go around the bends in the pipework from a good DIY or hardware store.
Peccavi, February 2009
