Saw this while browsing - seems to be the cheapest on ebay.
http://cgi.ebay.co.uk/XBOX-360-New-Sealed-w-Receipt-and-Warranty-UK-PAL_W0QQitemZ8239666435QQcategoryZ112848QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.co.uk/XBOX-360-New-Sealed-w-Receipt-and-Warranty-UK-PAL_W0QQitemZ8239666435QQcategoryZ112848QQssPageNameZWDVWQQrdZ1QQcmdZViewItem