At lunchtime today, just before checking the sender resistances, the fuel gauge was showing a tiny fraction over half full.
Jumped in the car tonight to come home, switched the ignition on, and the needle went up to about 7/8ths of a tank.
Oddly, it fluctuated a little, both up and down before settling. It's all very odd....
Do the senders have a visible metal track and slider arrangement, or are the arms attached to a potentiometer ?