why dont i have a fuel sender unit blanking plate under the seat?

Hi all, I have a '78 1.5LD. I've looked under the rear seat for the blanking plate covering the fuel sender unit but there isn't one. How do I get to and replace the sender?

Thanks!

Very strange…. there should be a black round metal plate held down with 3 screws, behind the drivers seat and in the middle of the rear bench.
only picture I've got and you can see it's near to where the carpet ends.
I have removed the black cover but you can see the 3 screw holes…
Do you have a picture of what you are looking at?