Quick update.
As I tried to add the cron jobs manually, either WP DB Backup or Jetpack the message at the top of the screen says "Successfully added the cron schedule jetpack_weekly" BUT there is nothing in the table.
If I refresh the page or login / logout nothing happens.
It's as if something is PREVENTING me from adding these cron jobs.
It would be helpful if some kind of error message was displayed - there is nothing in the error log file.