Robin - sorry but I do not understand this bit!
!If the 737 had had to move out of your way they'd have filed a report giving the ATCO a hard time for not keeping the required separation. The next person to upset them will be given a real roasting."
To answer the original question, move every time because (unless other circumstances dictate) it is far easier for GA to change course than CAT and no real problem to us.
So that makes the open FIR into CAS then.
If it is that difficult for CAT to integrate themselves with the rest of the flying fraternity in the open FIR I suggest they pay for those mode-S transponders so they can watch them on their TCAS.
Back to the original question. Comply if you want to but make it clear that you can't if circumstances dictate (weather or other traffic - would spoil your day if you turned onto the heading and collided with something else not on the ATC scope)