Lift the top off the cistern, and you'll see the flush syphon in the centre and the water fill valve either to the left or right. The fill valve will be adjustable somehow - press the float down and listen to the water flowing in. Now - can you see any sort of adjustment that will cause the float to shut of earlier? It could be in the form of a plastic screw or similar device to allow you to regulate the flow.
John, July 2009
