Thank you, Mike. I’ve tried Straight (unmatted) and both the Premul options, which make a slight difference to how the block looks before rendering but don’t change what happens on rendering. In all cases the video comes back in with a black background.
Other things I’ve tried are rendering with an empty video track underneath, and rendering in avi using the Lagarith codec, but neither of those make a difference.
Many thanks for your response
Jenny