Sending XML to a dot net web service

Should be a simple one, but im trying to post XML in string format using AJAX from JavaScript to a dot net web service (.asmx) written in VB, im struggling to get it to work quite how i want it to, i can get a simple string to pass into the web service, and return XML with no problems, but sending XML just doesnt want to play. Does anyone have a suggestion of a better way to send an XML document to a web service using Javascript. I have attached the function im calling that does the POST to the web service, i have tried changing the content type from text/XML to

the error i get from the response is a http 500 error :

System.InvalidOperationException: PageSecurityUpdate Web Service method name is not valid. at System.Web.Services.Protocols.HttpServerProtocol.Initialize() at System.Web.Services.Protocols.ServerProtocolFactory.Create(Type type, HttpContext context, HttpRequest request, HttpResponse response, Boolean& abortProcessing)

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######## Javascript AJAX function ##########
       function makeXMLPOSTRequest(url, xmlString) {
          http_request = false;
          http_request = GetXmlHttpObject(); 
          http_request.onreadystatechange = HTTPCallbackFunction();
          http_request.open('POST', url, true);
          http_request.setRequestHeader("Content-type", "text/XML");
        //  http_request.setRequestHeader("Content-length", parameters.length);
          http_request.setRequestHeader("Connection", "close");
          http_request.send("<?xml version='1.0' encoding='UTF-8'?>" + xmlString);
       }