I noticed that nagiosxi/html/includes/components/ccm/config.inc.php had the default server and credentials in it (I don't know if this is supposed to have the default creds or not), I updated it to the correct details and restarted apache, but this doesn't seem to have resolved the issue.
nagiosxi/html/config.inc.php also has the correct server and credentials in it for nagiosql.
It appears to be attempting to connect to the localhost still but I can't find anywhere else that these settings might live, to point it at the right server:
[Thu Apr 11 10:52:41 2013] [error] [client 172.31.121.248] PHP Warning: mysql_real_escape_string(): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2) in /usr/local/nagiosxi/html/includes/components/ccm/includes/common_functions.inc.php on line 378, referer: http://nagioshost/nagiosxi/includes/com ... -index.php