Thanks heaps Mike, but I took the sender unit down to the auto sparky to see what a repair might be worth compared to buying a new one, and he decided we might as well rip it open and have a looky inside. They are extremely simple inside, mine had no corrosion or damage that we could tell except for a really tiny wire that connects two points within the sender unit had totally rusted away. Sparky soldered in a new wire and its all go

I got it home, refitted the tank this afternoon, will wait till tomorrow to refill the tank (sealed the sender unit back in the tank with sealant as recommended by sparky) but the guage definitely moved with an empty tank
Cost $0, and now I know how to pull the tank out
