My original sender had been left in the tank for 10+ years and was a bit sorry...
The little Can that holds the thermistor was completely rotted and the reserve fuel level thermistor was dead. The wire float resistor windings were OK but the wiper (attached to the float) was out of alignment and everything was caked in rust. So basically not working.
The thermistor has one end connected to the reserve light (which has +v the other side) and when the thermistor is allowed to heat up (no fuel in the little can), the resistance drops to zero grounding the light and making it light. The Can is there to provide fuel splash proofing and has 3 small holes in the base to let fuel in and out.
If a winding is broken on the float gauge it can be replaced with Nichrome wire (resistance wire). I measured the wire at 34AWG thickness and probably about 8ft in length. The resistance range on my gauge is from 15 to 125 ohms (I have seen as low as 15 to 75 on forums). Basically get some wire and cut to a length that reads (in my case) 125 ohms if you need to replace the winding.
A new replacement for the old thermisistor is...
One end solders to the connection wire that attached to the board base of the Can unit and the other end can go to any earth. It is normally soldered to the top of the Can but as my Can was gone, I used a rubber filter plug and made a small hole in the top to take a wire I soldered to one end of the thermistor and then connected to the chassis of the sender.
Seems to work.
Last task is the new 7v regulator to go with the fuel/temp gauge.
Its a work in progress so will post later.