Thanks for your help guys - going to have a play with the settings. I know I'm not the only one with this problem so will post my results.
I've removed catalyst 14 and gone back to 13.11 and its not made a difference. Will probably try going back to 12 next if I can find a copy. These cards are mining on standard scrypt coins fine.
I'm using windows 7 btw.
Removing 14 and going back to 13.11 is not enough. If you've already remover 14 and installed 13.11 you need to uninstall 13.11, reboot, run
Driver Sweeper to remove all the junk left over, reboot, reinstall 13.11, reboot. That should eliminate the drivers as your problem.