Full of charm that Nobby fellow...
An overflow often happens when the float valve that controls the flow of water into a tank or toilet cistern is out of adjustment. The incoming water lifts a float on an arm as the water level rises - the arm operates on the valve that lets the water in and, as the level rises, shuts it off.
If the float is too high in the water the valve doesn't close properly and the solution is to adjust the float to be lower in the water to shut the valve sooner.
Sometimes the rubber in the valve perishes and a new one is needed - you have to turn the incoming water off to make this repair.
If you can get the lid off the toilet cistern you will be able to tell if the overflow is coming from there although some more modern cisterns overflow directly into the WC bowl rather than through a pipe to the outside.
If it's not the WC (which is easier to check) it will be the tank which will have a lid - be insulated and be more difficult to get to. Tanks often have a ball valve on a brass arm - you might have to bend the arm downwards with a bit of brute force.
You might consider working out the route of the overflow pipe into the property - that might give you a clue.
Good luck...
Peccavi
June 2009