I believe there are 2 sender units in your car so you will need to be sure which one is duff. It has been known on 202's and 208's (probably others) that low quality fuel can cause erractic fuel gauge operation, try a tank full of branded fuel and run some injector cleaner through as well as this cures mine everytime.
Andy in his post above has a very good point, and it has worked well on other cars, Do try a few tank fulls before diving in the deep end.
I can still help if needed
here is your tank with attachments