Fuel Gauge Problem

Has anyone had any experience of this fault and if so had it rectified. The fuel gauge on my in-laws 57 plate TD4 falls immediately from half full to zero and the car goes into a low power mode. Refilling the tank requires about 29 litres so I am guessing the tank is at half full, but because the gauge is reading empty a signal is sent to the ECU to cut power. The car has been "repaired" at a Landrover dealer but the fault has not been rectified. I'd be grateful if anyone can shed any light on this problem and how it can be properly fixed