Should it make any difference to the WCM client if there is a port forward envolved?
i.e. Alfresco is running on default 8080 on a linux box and the front end firewall port forwards from port 80 (external port) to port 8080 (internal port)?
Might explain why I am having some issues with the WCM GUI??
It seems that you were having issues with some browsers, but not others, so that does not really sound like a port forwarding problem to me, but here's something to determine that conclusively:
Remove the port forwarding, and try your experiment on the good & bad browsers. If you still see the problem on the "bad" browser, then clearly it had nothing to do with the port forward.
I think you'll see exactly the same errors on the "bad" browser, but this the definitive test.