Toilet cistern is overflowing?

I have a bottom filling cistern and the valve doesn't seem to be shutting the water off fully once the flush has finished, and the water's leaking out of the overflow pipe. There's nothing wrong with the ball (i.e. there's no water in it) but the brass arm doesn't seem to come up high enough in the water to shut the plastic valve off. Have tried adjusting the plastic screw in front of the valve but it's made no difference. I've looked for a replacement arm and valve but can't find one anywhere, and the bottom-filling valves I can find don't seem to use a ball float so I'm quite confused about what I need to get to repair the problem! Can you suggest anything? Thanks!