The first step is to test the gauge, wiring, and that the tank is earthed ok and all proves to be ok on the car.  The next step is to see if there is a problem with the sender unit.

 Once the top of the sender unit is removed, preferably through the removable plate in the floor if you have one, the sender unit itself is then removed, which may prove to be a bit of a challenge as some of the screws may need to be drilled out and replaced. The screws are the same as Meccano screws 5/32" Whitworth.

The biggest challenge in testing the gauge and sender unit is to hold them temporarily to test the movement of the sender unit, so a piece of wood is utilised. If you have removed the fuel tank, it is prudent to re-fit the sender and test the system before replacing the tank, remembering to apply a good earth just in case!

If you are using fuel resistant sealant between the sender and the tank this may be blocking the earth on the sender unit to the tank, so a wire needs to be added to the top of the unit back to the car.  Douglas Alderson, DA7C