I changed homepage to Right Sidebar only, and the video displayed correctly.
I also tried moving the video to the Left Sidebar and using Both Sidebars option, and it still displayed correctly in the Left Hand, even with Right Hand Sidebar active (this is how I’ve left the test site displaying for now, so you can see it).
Ideally I would still like to place the video in the right hand sidebar, with both sidebars displaying.
Is this any help in isolating the problem and achieving a fix?
I have turned off all but the most essential plug ins we use on the site, but none of that has made any difference.