Threaded Mode Diesel Fuel guage erratic low fuel and then half tank

[caretaker]

Hi Guys
My Vauxhall Astra J 1.7cdti, Hatchback, 5 Door, 2011 61Reg.

My Diesel Fuel Gauge is was showing few days a Low in fuel and then after a bit showing half tank and then again low fuel.

However on the same day in the evening i just filled in £20 worth of fuel just to be on the safe side, but since then i have driven 1.5 miles, but not noticed the error of the low fuel.

What could it be is it the Pump, Float Valve or the Sender Unit ?

Do you have to take the seat out to access the pump or sender unit or the fuel tank underneath the car.

Thanks.

I had posted this on the wrong section , so i posting it again here.

[Rusty2009]

I would say more the float but could be wrong.