Help needed...Faulty fuel gauge

[DROP IT - ZN]

Gauge showed 15l of fuel but after filling refused to move up...eventually filled up the tank but needle still never moved up but did move down as normal.
Now gauge shows totally empty but tank still has alot of fuel in it...
Did the cycle test on the cluster and all seems fine...even checked for loose wires at the sender unit beneath the back seat and that seems ok aswell

What could be the possible cause and what steps can be taken to remedy the situation?

 

[Dragonman]

Faulty Fuel sender unit in the tank. The rheostat on the sender is dirty/damaged. Not the wiring or the cluster, but the sender unit.