Some more investigation has proven that a new user not having posted to photopost can upload a photo, but existing users can't.
This seems to point to a permission/ownership problems that are eluded to in the post you directed me to below.
However that post doesn't talk explicitly about ownership, only permissions and I've done the permission side 4 times now just to make sure and have checked them all as far as I can manually. But I'm not sure what to check on the ownership side.
Check your user/group ownership permissions to ensure they match your web server.
I did notice that some of the files and directories are owned by daemon, presumably the Apache daemon.
Bit stuck as to what to do next but I'll keep looking.
And thanks for the help.