Hello Paul Wood,
The <?xml?> tag before the DOCTYPE of the page is called xml prolog and has been widely used in the near past by IE6 developers to trigger quirks mode without removing the doctype, or declaring non-standards compliance DTDs. This bug was later fixed in IE7, and that is why you experience skin problems under IE6.
We are sorry, but we do not provide controls that work in quirks or HTML 4 modes.
My advice is to try pure xhtml mode, and you will be fascinated by the tons of new possibilities for development.
the Telerik team