Thanks so much for your replies gents. It is greatly appreciated.
I have tested the sender unit and I think this is OK. Ohm meter reading ranges from about 5 (empty) to around 70 (full) and rises/dips evenly when float arm is moved at a constant rate either way. Both wires from gauge to send unit seem fine as well.
As mentioned by Rob, when checked for continuity between both wire terminals on the sender, I was only able to achieve continuity when the float level was close to empty. I assume this sends a return signal back to the gauge to activate the low petrol lamp?
Now I shall turn my attention to the gauge, which has not shown any signs of life in the slightest, even when ignition is on or wires are attached or taken off. It is happy sitting on empty. I have tried the workshop manual suggestion to supply the gauge with power and disconnect the tank sender wire. It makes no difference and stays on the Empty position.
I have removed the gauge completely from the car. If I hold it in its normal position and facing me, it reads empty and if I turn it upside down and facing me, it reads full. The transition between both when I am turning it is smooth.
On the rear of the gauge it has a B connection terminal (power), a T connection terminal (to tank sender) and a 3rd unmarked terminal connection. This third unmarked terminal supports the return wire from the tank sender which makes contact with the low petrol lamp.
Is anyone able to confirm the where the Earth contact point is for the gauge and if the fuel tank or tank sender unit needs earthing?