been out again and yes, the resistance does change with a meter on it...

and i think i have fixed it

sure enough, but fitting an earth wire under one of the screws to the battery -ve it starts to read!

so recallibrated it once again with the earth wire fitted and it now seems to be reading!

but i still dont think its 100% as its showing 1/16th over 3/4 of a tank, and the fuel is about 1.5" from the very top of the tank... so more than 3/4 full...

anyway, ill see how it goes!

cheers everyone that had a usefull comment