lego,
I had a mate who knows scoobys inside out look at it and he still couldn't sort it properly . Ended up connecting one of the wires from the tank sender unit to earth which got the gauge working again but it only reads halfway down i.e. when it gets to the 1/2 full mark the tank is nearly empty . The light still works . The mechanic said that we probably bypassed some faulty electronic device which changes the voltage so that the gauge reads properly .
Good luck .
Derek

 

cheers
john