Fuel gauge or sender unit?

[ChrisR]

My fuel indicator is playing up, sometimes works fine, sometimes doesn't stop where it should and when you turn the ignition off it rises instead of going down! Then after a while it'll go back down and work again for a while, then it'll break again.

I'm assuming this is the gauge itself that's knackered, but could it be the sender? How much am I looking at for the parts?