It's out, now I can't get the new one in!!
Thanks TerryS - (Mine's a 1995 110 V8 CSW). OK, I got the sender unit out -the potentiometer contacts were completely worn-out. I connected the new one and checked that the gauge indicates correctly when moving the float lever by hand, which it does.
Now I'm battling to get the new one in! Well, it went in first time, but the gauge didn't move when I filled up. So I pumped most of the fuel back into the front and side reserve fuel tanks, removed the sender unit, verified that it wasn't damaged, and replaced it. And again... And again...! But I can't get it right. I see (with a flashlight) that there are a number of places where the float can get snagged, especially with an empty tank. And when I'm having to do this blind with a few twists before it seats correctly, it appears that chances are slim that I'll ever get it right. And, of course, the float lever may have been inadvertantly bent during the now excessive handling.
I guess I'm heading back to square one!