these are a common fault, the only way to fix is to remove the tank and replace the sender unit, DANGER
make sure the tank is empty of fuel some have a drain plug under neath some dont, the screws holding in the sender unit always rust carfully using a hammer and small chisel tap the side of the screw they are not tight , if you cannot remove the tank cut a hole in the floor not pretty but its the only way good luck!
Daniel leaney, October 2008