erm doh, Thanks, 775 allowed plex to install and it is working fine.
mysql installs cleanly now too, but when I go to the web admin it says "Database connection failed to the server"
P.S. I'm not sure how the root perms got changed, I did have a share (smb) at the root which probably was a bad idea...
edit
mysql -uroot -p
entered password
Access denied for user 'root'@'localhost' (using password: YES)