Apologies Bruce, should have been https://<mailservername>/ews/Exchange.asmx. No, you can't get to it.
I've done a fair bit more research and experimentation since posting this up.
It gives back a 401 with "authorization" issue in the header.
The address I'm using is correct. I can log in over a browser. But using a Fetch with the same credentials (username/password) gives back the Authorization error 401 header.
Now.
Because /owa (the browser login for exchange) is set to "forms based security" (not basic or digest), EWS uses that security. Now I'm not sure exactly what it means, but the Basic and/or Digest code in nettalk doesn't work.
Am still very much in the unknown with this. But will plod on.