1. That this is a problem more prevalent in 2011-14 M3Ws. Since then, an improved sender has been fitted. He claimed that the older ones will show zero when there is about a third of a tank left; the newer ones when there are c.two gallons left.
2. That fitting a longer sender is unlikely to help by itself as the fuel gauge is calibrated to the factory-fitted device. ...
These are contradictory statements. If the gauge has been calibrated there wouldn't be a problem!