Thanks again for the help siran.
This seems to work very well.
Edit. Seems I was a bit premature there. I have a small problem in that if I try to replace the current front page backdrop it doesn't go full screen. It justifies to the left. If I add a new blank placeholder instead it works fine but replaces all the other pages with the background video too. Clearly there are other names used in the original background that I need to work through.
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