After much looking around I found the issue - I needed to put the external IP address of my router into Filemaker GO host IP address and not the one on my local network. I found the external IP address from my router using Aiport Utility (and it isn't the 10.0.1.** or a 192.168.0.** but the external on along the lines 18.104.22.168 for example). I set up port mapping of port 5003 using Airport Utility to point it to the static IP address of my iMac. The correct host I added was http://22.214.171.124:5003 and this worked fine.
I hope anyone else with similar issues gets it sorted too!!
It's never straight forward with computers is it!!