Tracking this baby down.
I did a diff on persistence and mysql-persistence and discovered that the mysql-persistence.xml file was corrupted. I opened it in Notepad which doesn't change anything. No wordwrapping etc, and it appeared to be fine. However, in diff, it showed considerable difference to the head of the "comes-with-openmeetings" persistence file. I tidied it up in the diff engine so now it looks ok. The DIFF program is Windows, "Beyond Compare 3" ... oh how I miss Unix.
It may well have been corrupted by either the download or the text editor I used (TextPad) or Gremlins ...
Carefully waded through both files, and in persistence.xml, the line create=true appears twice in succession.
Url=jdbc:derby:openmeetings;create=true,
create=true,
MaxActive=100,
Anyway, to cut a long story short - I replaced the persistence.xml with the mysql-persistence.xml, ran red5.bat, did localhost:5080/openmeetings/install and after a time - up it came.
So theoretically I'm now using the mysql engine.