Fuel gauge problems

i bought a series 3 landy just before christmas, ive been using it but have a problem with the fuel gauge. First it was stuck on full for a while and i realised something was wrong so i cleaned the terminals on the sender unit and it now ses 3 quarters. im pritty sure it has less than that in it so im taking no notice of the fuel gauge and am keeping it well topped up! Any thoughts on where the problem may be? New sender unit?

take the live wire off the sender and earth it. the gauge should read full. if it dont then the gauge is poss faulty. (poss wiring prob).
if it does then the sender is faulty.

Thanks for replying mate! sorry for the delay but im having trouble with my pc. ive tried that and it still aint working so i'll probably just buy a new sender unit but thanks any way.