Why does the cistern overflow?

My Villeroy & Boch toilet cistern flushes ok, but when it refills & the float mechanism cuts the water inlet off as it gets just below the top of the overflow pipe, after a while you can hear the water still slightly seeping in, which eventually spills over the top of the overflow pipe & into the bowl. Is there a seal inside the the float mechanism that has to be replaced, & if so, how do you get to it? I've tried adjusting the plastic screw but that doesn't seem to work?