what sort of sensor is in the tank. Some older style ones are a float on a lever arm working a rheostat so to get it to show full when the tank is full you just bend the arm a bit.
If it is anything like the sensors used with modern fuel injected motorcycles it is in a unit with the pump and senses the pressure of the depth of the fuel. The problem is that it works on a range of resistance from 9 ohms when full to 60 ohms when empty. It works on such low current that any little problem with connections can cause a variation in the resistance "seen" by the gauge. Caerbont told me that for the London Taxi cabs they finished up recommending an earth lead directly back to the battery to resolve issues. I think that in the case of the Morgan this signal is even passed through to th MFU and perhaps even the ECU (as is the coolant temperature) this gives scope for even more points where things can go wrong.
For the fuel gauge a tiny resistance of say 10 ohms somewhere along the way would ensure that the guage never reads beyond a little over 3/4 full.
