Ken,
Thanks very much for your help.
I was able to create a Private SQL server as part of my shared hosting package with OVH, and chose MariaDB 10.2.
Seemingly this uses the Barracuda format by default, as the error is no longer there and running the first php script showed nothing needed changing.
I was able to change the uft format with the other command. In order to do it I had to optimise the mdl_logstore_standard_log table.
I also tried changing all permissions for moodle and moodledata to 705 / 604.
I also stopped showing the Notifications icon in /admin/message.php.
One of these things has stopped the Undefined popups nag, but I am not sure which. I think it may actually have been the utf format change and logstore optimisation!?
Best wishes,
Hugh