My Ideal Standard Dual flush cistern keeps on passing small amount of water?

I have an Ideal Standard dual flush bottom entry cistern and it keeps passing a small amount of water continually even after the valve has shut off.
When filling the valve seems to leak around the top housing as well as filling down through the plastic pipe. As far as I can tell the valve is shutting off OK but when I push down to slightly overfill it, it seems to do the trick and stop the weeping.
Any ideas?