Most probably the sender - I fixed them both in the twin tanks of my series 3 ex-mil Landie.
Also had to have one changed in one of my defenders but I didn't do that myself (much harder to get to in the Defender). I think they fail quite often.
Might be an earth fault. +12v on sender is empty and 0v is full. check the voltage at the guage end and at the sender end to see if you have a short. Try 12 at the sender end to see if you get and empty reading.
