Take the lid off. Push the big ball down to stop the water flow. Then adjust the screw so that the big ball can stop the water flow for itself. The screw that you have to turn is near the end of the arm of the big coloured ball, keep pushing down on the ball so that you can turn the screw(s). (Adjust it to just below about an inch from the overflow pipe, thats where all you water is overflowing to). You might have to flush a few times to get it right.
Grant ( I hate plumbimg jobs!), July 2012