Remote Desktop Services Gateway "the logon attempt failed"

[emytjls]

I'm trying to setup a single server TS solution for remote access, I've
installed the Remote Desktop Session Host, Licensing, Gateway and Web Access
roles and installed a certificate from a trusted CA. From the Internal
network I can connect via web access and run RemoteApp programs however from
outside the network (or if I specify to connect through the RD Gateway) I can
login via web access but am unable to launch a Remote App program. On
attempting to do so I'm prompted for credentials as expected but on entering
a valid logon I immediatly get the error "the logon attempt failed". The
problem is exactly the same as that described here apart from the fact that
my setup has never worked
(http://serverfault.com/questions/8597/ts-rd-gateway-authentication-problem-the-logon-attempt-failed).
None of the resolutions suggested her work for me. Does anyone have any fresh
ideas?

Thanks