Spent a whole night fiddling with different settings as suggested and the best results so far are: Multiplayer single card - 50ish fps, action dependant.
Multiplayer Crossfire - 25 to 35 fps but graphics are very poor and and not smooth gameplay.
Campaign single player - Crossfire - 60 - 70 fps.
I thought there might be an issue with the clocks being a little different between the 2 cards, the XFX is overclocked as standard at 940 & 1150, the Sapphire being 900 & 1120 at stock settings, but changeing either in CCC graphics overdrive seems to make no difference whatsoever.
This forum has a pretty large number of members experiencing various problems with crossfire setups, surely ATI who promote the system quite heavily encouraging us to buy 2 cards instead of 1 with the promise of an improved visual experience should at the very least make sure the transition to Crossfire is as simple as possible, and yes, even foolproof.