Fuel Gauge reading

[RichiesWhiteS12]

Is there any common problems with the fuel gauge on mk2's.Mine does not go back when you turn the car off.It seems to like sitting at 1/4 of a tank even when putting in £20 of petrol.The gauge was reading 3/4 of a tank them about 9 miles of hardish driving was all most at 1/4 of a tank.Could this be the fuel gauge sender unit playing up or the gauge it self.Any help or tips would be great.