Either the float is too high - some floats can be adjusted lower in the water or a screw with a lock nut (often nylon) and be adjusted inwards to shut the valve sooner.
If this does not solve the problem then the rubber seal in the valve head may be perished and require changing - This will mean stopping the water and dismantling the valve. A completely new inlet, valve, float assembly may be had from a DIY store or plumbers' merchant but to replace it you will have to stop the water and drain out the cistern - else have a damp floor.
Try adjusting first...
Good luck...
Peccavi, July 2009
