usually corrosion issue at the sender end, be aware that disconnecting the sender can, if the corrosion is bad, result in broken connection, the two pins for the plug are hard wired to the sender and bent through 90' in the white plastic plug socket moulding. Because the externals are in the wheel arch area there's a lot of spray in THE area, the plug has a rubber boot which seals the whole. To test the wiring you disconnect and bridge the two connections, turn on ignition and gauge should go to full - inverse resistance reading - if there is corrosion then there is excess resistance hence false low reading; once plug is off you can see how green the connections are and possibly cut back to good wire and re-terminate
My SA van is the same at the moment and I haven't dared to remove plug and check it and clean up the contacts if needed (got enough electrical issues as it is). If the earth pin breaks that's an easy fix as the whole sender plate is the earth so you can connect to one of the 6mm studs, but, if the signal pin breaks it's sender out job to repair (modify/bodge) on bench or replace whole sender unit.
Edna's sender had a pin snap when I last disconnected it, after several cleans to keep it fully functioning, but I'm not using her now so will fix when the tank comes out
It's been a few years since we refitted your tank Pete
I've done just under 400km since filled so am carrying an emergency just in case 5 litre can and will fill up in next 50km, when next in Welshpool as Tesco is cheapest Super petrol station in the area, Llansantfraid is 23p/litre more expensive for Super than 95 and ain't cheap for 95Ron, the joys of living rural
