Error Message Feedback

Validating 

[ Add your message/question here.
  Follow the instructions at
http://validator.w3.org/feedback.html#errormsg ]

Dear Sir

I am trying to validate my document and I am now getting one error, which I cannot solve:

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Unknown Parse Mode! 

The MIME Media Type (text/html) for this document is used to serve both SGML and XML based documents, and it is not possible to disambiguate it based on the DOCTYPE Declaration in your document. Parsing will continue in SGML mode. 
-------------------------------------------------------------------------------------------------------------------------
I am using a custom DTD at www.cdesign.me.uk/HTML4plus.dtd

The page I am trying to validate is http://www.cdesign.me.uk/mendipmedia/index.htm

Have you any ideas as to why this is happening...

Thanks

Charlie