I can't include 'www' on Mapped Domains

I have domain mapping setup and 'Working' But I'm unable to map subsites to www.

To be specific. My Network is http://wiredstudios.co.uk

My Subsite is at http://wiredstudios.co.uk/sss/

I add the custom domain http://www.silverstreetstudios.co.uk

And when I go the site or check back to Domain mapping it's become http://silverstreetstudios.co.uk

Any ideas?

Thanks