So. I fitted Mondeo clocks (over two months ago) and filled the tank back then. It read F.
It got to slightly above about halfway down the gauge before it conked out, leading to having to reprime the fuel system (oh joy!). I didn't expect it to be that inaccurate
Now, does anyone know which way round the fuel sender works? Is it low resistance when full, and high resistance when empty? If so, an easy answer presents itself. Put a resistor in series with the sender to bias its reading DOWN. So it may never read more than half full when it's full, but the E reading - which is more essential - would be accurate.
Alternatively, if someone could suggest a different sender unit that might work more accurately that's also a possibility... Or, an adjustment to the clocks. Maybe there's a potentiometer on there somewhere to adjust the range it expects?