My gauge (1996 diesel 2wd freda) was showing full fine. But it never went below a quarter (which was interesting when I ran out in the middle lane of the M4, with cones on the hard shoulder, going uphill...

Anyway, I made a new earth connection. No change.
So, bought a sender unit off ebay (2nd hand but looked OK). Before installing it properly into the tank I wired it up to the existing plug and my new earth to test it fixed the problem. And it did. The gauge went right down to - and beyond - empty when I put the float in the DOWN position.
Then I flipped the float right UP to simulate full. Result - nothing. The gauge stayed on empty (igintion was on). So I tried making a new earth connection. Still no joy.
Assumed this 2nd hand sender was duff, so wired my old sender back in. And now that won't move the gauge off empty. Disaster.
I can only think that I may have somehow blown something in the gauge when I tested the float manually - perhaps the gauge can't cope with the float moving quickly.
Can anyone advise? Anything else I could test to identify where the fault lies? I don't see that there is a fuse just for the gauge.
Thanks.
Sam