I've seen a few examples of how to setup Emacs to use Basic Auth proxies - is there a way to configure it to use an NTLM proxy?
If anyone needs to answer this question, here's how I solved it:
Install CNTLM and configure it with your NTLM creds.
Set environment variables HTTP_PROXY & HTTPS_PROXY to the localhost presented by CNTLM
that should sort it