Discovery 4 fuel empty warning

Hi All
My fuel gauge started showing empty after I filled up this was an intermittent fault but now permanent problem. Tried using diagnostic but showed nothing.

Tried Google but too many answers could this be a disconnected fuel sensor and how is it fixed.

Need brief details as do not know where the sensor is.

Many thanks as always Yvonne

I have read that rodents like to get on top of the tank and chew the sender wiring, only way to access the sender is to drop the tank.

Thanks big job then