Getting 404 error when trying to access new site in MU

Hi
I have setup a Multi Site copy of wordpress as a test system. The base site is working OK http://www.astellan.com but if I try to create a sub-domain and then access it I get a 404 error. i.e. cctv-sales.astellan.com

I suspect that I haven't set something correctly but not really sure where to start looking.