I had one like that on a Discovery.
Once below approx 1/2 tank, it used to drop like a stone.
Do your other gauges work correctly? If not, you may find there is a poor connection to an 'earth' or common return point behind the dash panel.
If they do, then check all the connections (that are reasonably easy to get to, so: mainly behind the rear panel) are clean and make good contact.
If all else fails you may want to lift the sender unit from the tank and check its linearity.
Cheers,
