Hi Chris,
It ended up not being permissions on one case for definite (attachments in PM's to be specific being posted on open forum).
On other cases it seemed to be permissions set to No much further down the tree which seemed to trip it up, but I can't be 100% certain as some time has passed. If it happens again then i'll post back.
At the moment, Global and Node are not showing a problem so I'm awaiting the user's permission to get into their account.