Actually I fixed it in the end by getting a new cable, it turned out that for whatever reason my cable was not up to the job.\
I found that a cheap £5 one piece thunderbolt male to hdmi male cable off ebay did the job well and it was my two piece - thunderbolt male to hdmi female attached to a standard HDMI to HDMI cable which caused the problem.
Seems like HDMI can be quite fussy and either it works or it does nothing, no in between.
So my fancy mac now works great with my cheapo alba hdmi tv i paid £25 for and carries both sound and video!
(I don't think I'll go back to windows if I can help it, not after using a mac. Once you've had a mac you realise what a pain in the a*s windows is .. I hardly ever feel like punching my mac )