It should generally either work or not work, but according to Wikipedia...
http://en.wikipedia.org/wiki/HDMI#Cable_length
Quote:
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Originally Posted by Wikipedia
Long cable lengths can cause instability of HDCP and blinking on the screen due to the weakened DDC signal which HDCP requires
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Can also occur if the cable is faulty & is causing HDCP issues - doesn't have to be caused by a cable which is too long.
Just a thought, as although the OP has changed TV, & has changed V+ box, there's no mention of changing HDMI cable.
Then again... Northern Laddie has a similar problem, with the same TV, and changing HDMI cables made no difference.