Windows 8 - RDP 8 - Public Network - RD Gateway problem

Hi

I have a Windows 2008 R2 Remote Desktop Gateway Server.

I take a laptop and format it with Windows 8. I then join it to a 'Public' network. I then use RDWEB and connect to the RD Gateway server no problem.

If I then put that laptop onto our domain, from the 'Domain Network' it all still works. However, if I then connect the laptop to a 'Public' network, when I connect to the Gateway I get the following message: 

"This computer can’t connect to the remote computer because the Terminal Services Gateway server is temporarily unavailable. Try reconnecting later or contact your network administrator for assistance.”

In short, with RDP 8 I get the above message if I use a domain joined machine on a private or public network to connect to a RD Gateway.

Any ideas??

Thanks

Andrew