Not needing help as I found a work-around.
You Tube have changed their "Embed" again. it has an iframes html tag and produces a blank post when used as per normal WordPress protocol.
I ended up finding an old video I had successfully posted and, cut and pasting, changed the old you tube url code (the bit after the "v=" for the new url code and that worked.
It seems a bit long to do something that was simple but at least it is a way of getting it to work until the WordPress Wizards give us an easier way of doing it.