Quick update on this one - managed to spend a few more hours on it today, and made up the temporary link-lead to allow power to be supplied to the in-tank fuel pump, whilst leaving the sender units disconnected.
This meant that I could now run the engine, whilst substituting known value resistors in the place of the senders, by carefully inserting them into the end of loom connector.
Contrary to other posts in this thread regarding possible algorithms and the fuel gauge remaining fixed whilst filling up, I can now confirm the gauge does in fact move both up and down with the engine running or with just the ignition on, if the resistance is varied.
Once again, having obtained the anticipated results at the gauge with the car's tank senders removed from the circuit, it's looking very likely that the fault lies with one or both rheostats.
However, I'll test it for a few days with resistors in place, just to confirm the eratic needle behaviour has stopped, before having a look inside the tank.
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