How do i Undo this union!? Pls Help!

Guys,
 
I need help. I am changing the fuel tank on my diesel 110. All good so far, but I cannot get the union undone that attaches the sender unit to the pipe taking the diesel up front.
 
I know unions need 2 spanners, but I cannot work out which nut to turn - the one nearest the fuel tank, or the one attaching the fuel line. I am anxious about getting it wrong as it would mean either a new sender unit, or a new fuel line (with lots of nasty unions) or both!
 
Some guidance really appreciated before I total elements of the car that were just fine!
 
Thanks you very much.

Oh yes, and also, when I put it all back together again do I need to put gasket sealer around all the faces on the sender unit, the union reconnect etc or should they mate fine?
 
Many Thanks
 
Argoman