Strewth. It's not an Exchange server running at this home Andrew, it's elsewhere. It works fine from his Orange broadband ergo it's not an issue with port forwarding at the server side. People rarely run Exchange, which Outlook Anywhere is a front end for, at home.
People who run Exchange servers tend to not mess up things like 'port forwarding' and chances are it's not using port forwarding but static NAT and a virtual IP translating everything going to a public IP to the private IP address of the server in question.
If his software firewall were blocking web browsing he'd have had a rather troubled time posting this thread wouldn't he? I think we're probably safe to ignore that one, and yes I know what software firewalls do, it's why I don't use one of the horrid things
I do hope when you troubleshoot at work you read more carefully. The Exchange server is somewhere in Internet land, it works fine from his Orange DSL, it doesn't from his Virgin Media cable. It's not that he is hosting it locally, sane people don't set up a personal Exchange server on their broadband.
Anyway onto the point.
Irish, either get a packet trace or maybe get the techies who operate the server to see if you are actually reaching it using the Virgin Media cable connection. You mentioned getting a password prompt but you didn't mention at what point, are you seeing the initial screen requesting the logon? Are you giving the full domain and username info if you do according to the prompting? If you're going over ordinary http have you tried going over https?