Hi Nick ( good to see you at Stoneleigh ).
My fuel sender is earthed from the sender unit into the main earthing system within the loom, it shows the fuel level when ignition is off, tho I,ll be honest, my system might also be confusing itself ( electrics confuse me
) as I also have a battery isolator switch which when turned off still shows the same level on the fuel gauge.
I,m not sure if its needed but I also have my fuel filler cap/aerostyle cap earthed direct to the chassis too.
cheers
andy