indy 9 and ISA server

I'm having problems writing a simple client application because of my
ISA proxy

my code:

   httpClient := TIdHTTP.Create(self);
   proxyParams := TIdProxyConnectionInfo.Create();

   proxyParams.BasicAuthentication := true;
   proxyParams.ProxyServer := server;
   proxyParams.ProxyPort := 8080;
   proxyParams.ProxyUsername := username;
   proxyParams.ProxyPassword := password;

   httpClient.ProxyParams := proxyParams;

   httpClient.Host := host;
   httpClient.Port := 80;
   httpClient.HandleRedirects := true;

   httpClient.Get(URL,HTTPResponse); //HTTPResponse being a TMemoryStream


I get a response from the ISA server:

HTTP/1.1 407 Proxy Authorisation Required (The ISA server requires
authorisation to fulfill the request. Access to the Web Proxy service is
denied.)


I've been looking at the headers that Firefox sends when it opens web
pages (using LiveHttpHeaders extension) and this is a normal response
from the ISA server. The difference between Firefox and my application
is that Firefox then resends the request with some authorisation
headers, gets yet another 407 reply, resends it yet again with some more
authorisation headers and finally gets through the proxy server.

Ideas anyone?

thanks


PS: Indy 9.0.18 & Delphi 7