Think I have figured it out now.
For future reference I deactivated the plugins by renaming plugins folder in FTP which then seemed to sort the issue and allow me into the /wp-admin.
I then reactivated the plugins in FTP then de-activated all plugins in /wp-admin then activated one by one. I found the culprit was the Custom Post Type UI 1.1.2. I remember trying to turn ‘Has Archive’ to true and add a new slug then, a few minutes later, changing this back to False at which point the Warnings started to appear.
All appears to be OK now.