Well I seem to have serendipitously came across a solution to my problem. It turns out that it is my TV to blame for my blank screen and probably MP freezing on me. I have discovered that the rear HDMI ports on my 6 year old Sony TV behave differently to the side ones. If I connect my HTPC to the rear ones I will get a picture but as soon as I put the TV on standby or even switch inputs the screen becomes blank when I return to the input the HTPC is connected to! This occurs with both rear HDMI ports. Strangely my Chromecast does not exhibit this behaviour when connected to these ports. I have no idea why my TV is doing this and can only imagine that there is some fault in the TV's hardware / firmware. I now have my HTPC connected to one of the side ports and I haven't been able to reproduce either the blank screen or MP freezing issues 
As part of my troubleshooting I followed the advice on this website http://www.avsforum.com/forum/26-ho...des-solve-bitstreaming-issues-ati-5xxx-s.html
Doing this hasn't fixed the issues with my rear ports but with my HTPC now connected to my side port the image seems to come up faster than it has ever done in the past. Not 100% sure if this solution is responsible for this behaviour but it is an easy thing to try.
Thanks everyone for all your advice.
As part of my troubleshooting I followed the advice on this website http://www.avsforum.com/forum/26-ho...des-solve-bitstreaming-issues-ati-5xxx-s.html
Doing this hasn't fixed the issues with my rear ports but with my HTPC now connected to my side port the image seems to come up faster than it has ever done in the past. Not 100% sure if this solution is responsible for this behaviour but it is an easy thing to try.
Thanks everyone for all your advice.