Take the lid off the cistern and operate the flushing mechanism.
Observe what happens
Do it several times
Notice how levers move with the incoming water and a rising float.
Pull up on the lever before the water reaches the high point and see the water stop flowing in.
Pay attention to the outflow (overlflow) pipe - it is best to have the cistern stop filling when the water is half an inch (12mm) below the level of the overflow.
Look for a small nylon screw adjuster on the moving arm - this is the piece that presses on the valve to stop the water entering.
Screw it inwards towards the shut off valve. Continue adjusting and flushing until the water level is satisfactory.
Most importantly tighten the locking nut on the adjusting screw.
Replace the lid on the cistern.
Go in peace.
Peccavi
March 2013