[Greg Spark]

The video mute button was not on for the track BUT have discovered the problem thanks to a single post buried deep in a thread (I had searched and searched before I posted):

There is an option to “Mute all video” in the Video Preview preferences . I never turned it on but it was on. Unchecked and problem solved.

I had been in correspondence with Sony Support and they never suggested that as an answer.

Thanks for your replies guys. Great forum!

Greg