fuel light mystery

hello all,
I have a 900ie which has a problem with the fuel light in that it always goes on when I turn on the ignition.... whatever the fuel level.
I can disconnect the block connector just below the tank on the rhs and the light goes out but I would rather fix the problem. I have noticed that there is a large nut on the rhs of the tank just near the hinge and i am wondering if that is the way to the fuel level sensor.
If I do manage to take it out, is it easy to fix? do I need to get a replacement? Can you offer me any advice?

Thanks!