I've just had some success sorting my problem out and I hope it will translate to success for anybody else with the same issue.
I couldn't turn CrossFireX off; I could select the option, but it just bounced back and was never applied.
I had "upgraded" from v12.6 to v13.1. In the good old days upgrading used to often cause problems, but in more recent years we've got used to driver upgrades just working so I thought nothing of it.
Then I decided to go old school. I uninstalled all AMD/ATi software, went back to the default drivers supplied by Microsoft with Windows 8 and restarted the PC.
Then I did a full clean install of the v13.1 drivers. After several reboots, some where my screens just sat there looking black (on, but no display) all is well. My power button is set to "Shutdown" so although I had no display I could still turn the PC off safely.
So a bit hairy, but it got there in the end. My machine is working happily with all three monitors, the latest drivers and CrossFireX can be turned off for work and back on for gaming.