Just thought I'd post that I now got this solved, FINALLY!
After installing, re-installing, deleting, checking about everything and without success, I went into the AMD control panel and had a look through the video settings, and just to be absolutely sure I'd tried everything before returning the CPU I unchecked everything (De-interlacing, edge enhancement, de-blocking and so on, and the final piece that broke HW acceleration on MPEG4-AVC-3D.... Enforce smooth video playback
So now I'm all good, and damn you AMD for making that setting a default
Cheers and thanks all your help!