Draytek tech support suggested firmware recovery to me but I don't think it's a firmware corruption - it's more like a bug in the firmware as it's been reported by several posters here. Anyone from Draytek reading this?

Does anyone know if this issue is in 3.7.1 ?

I did flash the router to 3.7.3 before taking it to site.

After speaking to Draytek support I have found that there is another tick box that need to be tick to allow access to remote web access

In System Maintenance > Admin Settings
Tick Enable Admin Login From WAN

Now I have access to the web GUI

I'm 99% certain I'd enabled that. My issue was that (from the WAN side) I was presented with the login screen but it would not accept the credentials. I knew they were correct because I could login from the LAN side.

I had the same thing, but was able to telnet in to the router but not the web GUI hence I knew the username and password was correct.

It would just go back to the GUI screen with the correct username and password but tell you you entered incorrect information if you tried a different username/password.

Once I ticked the enable admin from WAN it worked.

I'm running 3.7.3 first release datd 20th August 2013 as downloaded from draytek.co.uk

Thanks. For info for anyone else, the setting is here:

System Maintenance/Administrator Password/Enable Admin Login From Wan