I see it way too often... A site where the www of the domain simply does not work. In some cases, the www version goes to a 404 page or a parked domain page, whereas the non www version goes to the proper site.
A Google Webmaster Help thread has a Googler saying this can be very bad for your site's performance in Google.
Gary Illyes from Google said, "it may very well affect your site in a negative way. I strongly suggest you to fix it." Gary was referring to how the non www version of his domain went to a parked domain.
Do you know why your domain name is parked? http://realclayovens.co.uk/
This is a very bad user experience and may very well confuse our algorithms as well. If you want to serve content only on the www version of your site, it would make sense to 301 redirect realclayovens.co.uk to www.realclayovens.co.uk
This may be common sense for most SEOs and some webmasters, but sometimes it is good to have this documented as proof.
Forum discussion at Google Webmaster Help.