FGW staff travel policy is pretty clear and it has recently been commented upon in the in house magazine (a very similar situation to the one the OP references!)
Unless your travel pass states 1st class, then you aren't allowed to travel 1st!!
Fair enough, if its quiet and the TM gives you the nod and invites you to sit there then do so, but play the game, at the end of the day if someone complains about your behaviour then its not just yourself you will be landing in it.
As for each person occupying a section of four each - why, if they want to chat to each other, sit round the same table!
I've experienced the same on the bus, two people get on together chatting but then sit on opposite sides of the aisle, at the window seats, shouting across the aisle to each other, Morons!!