I suspect you changed your httpd.conf to say:
ServerName http://welford-costelloe.com
That's not what I meant. :-) In your httpd.conf, use:
ServerName welford-costelloe.com
and then in your browser, type:
http://welford-costelloe.com
Note: "welford-costelloe.com" (or whatever other host name you use in both places) needs to resolve (via DNS or the client's "hosts" file) to the IP address of the server.
--Dean