Richtext in a SOAP request

Soap webservice, referenced using a .net 3.5 service reference

Text Encoding is utf-8

The field type is a string

<s:element minOccurs="0" maxOccurs="1" name="Content" type="s:string" />

When I try to post richtext (hexadecimal characters) in the string property, the XML breaks.

System.Web.Services.Asmx Error: 0 : Exception Details:
System.Web.Services.Protocols.SoapException: Server was unable to read request. ---> System.InvalidOperationException: There is an error in XML document (1, 13434). ---> System.Xml.XmlException: '', hexadecimal value 0x0B, is an invalid character. Line 1, position 13434.
   at System.Xml.XmlTextReaderImpl.Throw(Exception e)
etc.....

I have tried SecurityElement.Escape, HTMLEncode plus a few other encoding methods to no avail.

Do you have any suggestions?

Is there any way to force a property to use CDATA in the XML request/responses?

I would prefer not to have to manually handle this at each end point.

Thanks in advance