See pic below for tank diagram
The sender is number 10/11/12 and has a float attached at the bottom which goes up & down with the fuel level. This movement is turned into an electrical signal..which is fed to your fuel gauge.

They are easy enough to change: but you have to remove the petrol tank as it fits into the top...hence why I said if you are happy to run with it like that then leave it unless your tank is rusty and needs replacing aswell. Do both jobs in one go to save messing around and labour costs. New tanks are around £60-70 ish. Senders guessing around the same: but not sure if you can buy new items: might have to be 2nd hand. If you do change the tank/sender, then get the brake lines checked along that side of the car: they run along the top of the tank and it's a tank-out job to replace them if they need doing. Get them done at the same time also

BUT

Ensure it is not an ELECTRICAL fault first..as Safechav is trying to establish..as you don't want to be changing the sender if it isn't faulty.