Not receiving email from Scheduled Reminder for Activity
OK, answering my own question after much investigation. This was a bitnami civiCRM stack and the crontab to run Job.execute was running as the bitnami user (our error), but needed to run as daemon.
That is documented on bitnami's site:
https://docs.bitnami.com/bch/apps/civicrm/configuration/configure-scheduler/

Strange behaviour as a result of this - cron seemed to be running fine, but it clearly didn't have permissions to send emails.