Early pre-cat fuel sender unit

I have a dead fuel sender unit in my early pre-cat...sits on the left hand side of the tank not top right. Ordered one from a well known specialist which fits fine but the resistance readings are incorrect i.e. It reads full, responds to 3/4 and then goes straight to empty. Gauge is has tested ok.

Can anyone point me in the right direction for a replacement? I have done some research and it appears that a Land Rover one may be the right spec but Ino specific part numbers..