The DNS server has been configured to disable the 'www' URL. If you type in an address with 'www' or click on a link containing this you get redirected through to the 'not found' response as set up by your ISP. So no header is returned technically.
I did actually want to return a 404 as if nothing else this has proved frustrating for users. The thing is I couldn't work out how to do this (any ideas?) Current rankings suggest that the 'www' links aren't counting towards the non-www and all of my pages seem to rank differently to how they did but I don't want Google to start combining these.