http://portforward.com you must tell your router where to send the request on port 591, the local computer hosting the database.
Googel "iwp port forward site:forums.filemaker.com"
Please note that there is a newer release of FileMaker 12 available. You can go to FileMaker's download page and download a free updater to update your copy of FileMaker Pro to either 12.0v4 (Windows) or 12.0v5 (Mac)
Thanks for the advice David. After more fiddling around and reading more about port forwarding, we worked out that its definitely an issue with the router and my inability to port forward 591. It may also be due to an inbuilt firewall with our ISP on port forwarding (something I read) as we are using 4G connection, not a lined service. With direct internet to my PC via a USB 3G mobile broadband, I was able to use IWP. I'll continue trying to resolve the port forwarding issues with my router, but this is a temporary fix.