flarkit, have you tried running the -diag switch after the gw.exe?
If all is well there, it may be gw.dat-related or even application firewall related.
I had an extremely strange scenario recently where I couldn't run GW from one of my networks (but it worked from anywhere / everywhere else) and others were using the same network fine.
I ended up uninstalling and upgrading my Firewall application software.
In all the time I've played GW, it seems some sort of information is stored in the GW.dat / cache file that is to do with your connection/s.
I've had several funny problems, I am very mobile and only play GW on my notebook, so ye....