Ok, so I have taken the sender unit out and dismantled it. Wow, what a load of crap I found trapped in there. Mainly corrosion. From what I could see there was a big blob of rust that was wrapped around the small thin wire where it attaches to the plastic insulation block where the wire exits out from the unit. This blob of rust had broken the fine wire,( just like yours Pete ).
I carefully cleaned all the rust / scale out and very carefully cleaned the blob of rust from around the small fine wire.
Next I stripped a section of old original wiring I have, and extracted one strand of wire. I wasn't to sure if I was able to do it but I managed to very carefully solder a new section of wire in. Next I cleaned up the contact on the slider that rubs up and down on the resistance coils and rebent it in to make a better contact, as it was not making contact all the way up.
When all this was done I tested it on the bike with success, the gauge now works again.
I refitted the casing around the sender unit and retested the unit, still working fine. Put it back in the tank and sat the tank back on the bike, reconnected the wiring and tested again, low and behold it still works.
Thanks again to all for your advice.