CiviEvent - front end registration error page - permissions or something else
OK, found out what this was. There is a new check box in CiviContribute Components Settings 'Enable Access Control by Financial Type' In the users system this was checked. I don't know whether an administrator accidentally ticked that without knowing what it did or whether an update process ticked it.

Anyway, this being ticked with no civi ACL obviously locked everyone out of being able to access the financial types and therefore broke the front-end event registration process.

Un-check that field and save and the system is fine once again.

Hope this helps anyone else in this situation.

(PS - it looks like super users within Joomla are somehow treated differently which threw me initially the normal civi ACL checks obviously don;t apply to Super Users - hmm)