Following upgrade to 1.4.4.3 the gallery (www.ormondhistory.ie/zenphoto)is unreachable, as is the admin page.Tried to restore from previous backups but getting errors on all of them.

Without knowing the errors there is little we can do for you.

Attempting to access the admin page or the gallery returns this - HTTP Error 500 (Internal Server Error):

You really need to look into your server error log. Otherwise we are guessing. It could be permissions, wrong htaccess file and other things.

most likely file permissions. Some servers are very particular on the permissions they allow for scripts to be executed.