Had a think back to when this started and think I have it.
It all used to work OK, then my ACD had to go for repair (dirt in the Panel) while it was away I used a Samsung Monitor
plugged into the DVI port.
Since the return of the ACD I have had no Brightness slider nor F1 and F2 key brightness function.
Monitor and keyboard have been tested on other macs and they all work as they should.
It is as though my Mac Pro thinks the Samsung is still connected, very strange :/