How do I get 'www' to show up with multisite and WPMU domain mapping.

I am using multisite at http://www.the4propertygroup.co.uk.

I have installed the WPMU domain mapping tool.

My problem is that although both the www and non-www versions of the4propertygroup.co.uk display the site, on the sites within my network, such as flatmates4london.co.uk, ONLY the non-www version works. If you type in http://www.flatmates4london.co.uk - you see the registrars holding page.

Can anyone tell me if this an issue with the plugin, WP or my DNS and what I can do to solve it.

Thanks