Thanks Mike, I knew somebody would be awake.
Looks like the second wire inside the tank has been removed at some time. Just a broken connection inside the sender and an empty screw down the bottom. I thought it may be for a low fuel light but didn't know if the car had one of them. From the diagram I can see it does.
I've lengthened the float and that has altered the guage a bit, but seems I need to lengthen it a lot to get anywhere near accurate.
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