I would have deleted my original post and question, but incase anyone runs into the problem I did, I will leave this here for now...
I installed some freeware network software called Hamachi on both a PC and a Mac..
I then started a new server within Hamachi on the PC, and it then gave me a new IP address beginning with 5.192.x etc
I got the Mac to join that server on the new IP address, and then when I start the game, just use the new IP as opposed to my normal and it works!!
I hope this may help anyone wanting to play this AWESOME game!!