There's absolutely no reason why this shouldn't work OK but first can I just ask, exactly what did you do to connect when you were on TalkTalk?
So you are using a Mac, by MS Remote Access, do you mean Remote Desktop Services? That's done via the application downloaded from Microsoft via the Apple App store now. If so did you just start up the RDP client application, select the pre-saved IP address for your work PC, click connect and it worked or did you have to do something else first (ie connect to your office network via a VPN tunnel)?
My gut reaction here is that your office firewall is only allowing incoming RDP traffic from particular addresses (which would be a very good thing otherwise the world and his wife will be trying to connect to your office PC), once you moved to VM, your 'source' address has changed and that needs to be reflected in the settings in your office equipment.
Your IT person should at least be able to check if there are any attempted incoming connections.