Having been using the car for almost a week now (driving, stopping & starting, parking on different inclines, varying ambient temperatures, etc.) with fixed resistors in place of the tank senders, the gauge has behaved itself with absolutely none of the eratic behaviour seen previously
Last week, I ordered a potentiometer (500 ohm) and today I swapped the fixed resistor over for this, just to see if I could make the gauge move to any desired position without any odd movements. Once again, with the ignition on, the needle moved up & down exactly as predicted, from full to empty or any chosen position in between.
So I think I can now confidently confirm to other members that a correctly working 75/ZT fuel gauge system DOES indeed show any change in fuel level if the ignition is on. You can forget the idea of complicated algorithms, the system works exactly as expected if the resistance is varied - although the needle does move quite slowly, taking maybe 30 seconds to travel it's full range.
Hopefully, I'll get chance to have a look at the sender circuit boards tomorrow....