Ewan you need to set your MagicInfo pro server on a static IP address and then tell the clients to look for that IP address. The setting is on Tools, Option, Network Connection set to using Direct IP rather that server name.
This will create a 'tunnel' through your network without having to worry about Firewalls etc.
Remember you have to set the client to point to that server IP address. If you go to the client, press escape, then right click on the globe (bottom right hand side on the Windows bar by Time), Setup.
Bill