HOORAY (well almost)!
The latest catalyst drivers, version 12.4, have solved this problem ... BUT, they have introduced a new problem whereby the HDMI driver is disabled when the TV is powered off! However, there are reports that the beta for 12.6 (don't know what happened to 12.5) has solved this problem. Personally I've solved it temporarily by uninstalling the AMD HDMI driver and rverting to the standard MS one for the time being.