Embed on Moodle
OK. It turns out it was because I changed from Boost default to plain.
I found a fix mentioned in another post: https://h5p.org/node/38304
Since the last install (I think) the H5P module doesn't seem to expand to fit the editor body area in Moodle.
When published, the video itself displays fine at 100% of the body area.
I have attached the screenshot.