Update on this is that it appears to be Better WP Security Plug in that is causing my scheduled Backups to not work unless I am logged in. Just disabling BWPS did not work, so I activated it again and switched everything I could off in hte plugin and then I got my scheduled backup to work. Struggling to find the setting in BWPS that is causing the problem but if I find it I will let you know.
If anyone else has had any issues with BWPS Plug in and WordPress Backup to Dropbox and solved them, I would appreciate some info.
All the best, David
