Swift Fiat Ducato

i have a 2011  fiat Ducato 2.1 carrying a swift 622 motorhome. We went away and didnt use the vehicle for a while. we have a battery conditioner but whilst away we had a power cut so the battery was flat. Once charged its now fine. However the tank was half full with diesel but when we came back it showed no fuel and the warning light was flashing. i filled it up with fuel but as i thought it had about half a tank in already. The light will not now go out. Does anyone recognize this problem? Before i change the sender unit on the tank i want to know if anyone out there can help from past experience?

Any help would be appreciated