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If you are certain the sender is working OK when removed from the tank - then logic would seem to say something is happening when its inside the tank.
Could it be the float is jamming when immersed in fuel. It may be worth removing the unit from the tank trying submersing it in a container (paraffn or such would probably suffice) to see what happens. It may involve extending the cables with a temporary connection but that shouldn't be too difficult.
Better still if it could be rigged on the bench using a multi-meter to check it resistance may actually be as good.
Currently in madnoel10's garage:
Honda Civic 1.4l