Ati5570 issue

I have a ATI 5570 graphics card and just bought a HDMI cable to go from my PC to my TV screen (I have been using VGA up until now).
It works fine but it's stuck on 1080 rather than 768 which is the maximum my TV has, The Catalyst Control Center & the generic windows display won't let me change this either...
Any tips?
 
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