401 unauthorised error when using WebRequest - asp.net

Question

Am trying to create a simple page that connects to an external website, logs in, and then passes a bunch of parameters.
When I run the page I get a bad request 400 error and when I check using Fiddler I can see there are both 401 and 400 errors being returned. Under 'Auth' in Fiddler I see:
"No Proxy-Authorization Header is present.
No Authorization Header is present." < Is this relevant? As when I test using PHP cUrl the page can log in fine and Fiddler says the same under Auth.
Dim UserName As String = "testusername"
Dim password As String = "testpassword"
Dim siteCredentials As New NetworkCredential(UserName, password)
Dim URLAuth As String = "http://service.someurl.com/process.xml"
Dim postString As String = String.Format("customeremailaddress={0}&customername={1}&referenceid={2}&languagecode={3}&expirydays={4}", customeremailaddress, customername, referenceid, languagecode, expirydays)
Dim postBytes As Byte() = Encoding.UTF8.GetBytes(postString)
Const contentType As String = "application/x-www-form-urlencoded"
System.Net.ServicePointManager.Expect100Continue = False
Dim cookies As New CookieContainer()
Dim webRequest__1 As HttpWebRequest = TryCast(WebRequest.Create(URLAuth), HttpWebRequest)
webRequest__1.Method = "POST"
webRequest__1.ContentType = contentType
webRequest__1.CookieContainer = cookies
webRequest__1.ContentLength = postBytes.Length
webRequest__1.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.1) Gecko/2008070208 Firefox/3.0.1"
webRequest__1.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"
webRequest__1.Referer = "http://service.someurl.com/process.xml"
webRequest__1.Credentials = siteCredentials
Try
Dim requestStream As Stream = webRequest__1.GetRequestStream()
requestStream.Write(postBytes, 0, postBytes.Length)
Dim testcapture As String = requestStream.ToString
Dim thing As String = "stop"
Dim responseReader As New StreamReader(webRequest__1.GetResponse().GetResponseStream())
Dim responseData As String = responseReader.ReadToEnd()
responseReader.Close()
webRequest__1.GetResponse().Close()
Catch ex As Exception
Lbl_ConnTest_error.Text = ex.Message
End Try
Seems the login credentials are not being passed what am I doing wrong? I am obviously using vb in an asp.net application but am connecting to an XML file held on Linux Apache, does this have any implications at all?
Fiddler appears to be doing strange things for me now so if anyone else can see anything using Fiddler a link to the test file is here:
http://www.filemanage.co.uk/Pubs/Default.aspx