Ubuntu Networking :: VNC Not Tunnelling Over SSH Correctly 9.1

I started another thread on this (URl...), but the topic was not specific to what I now know the problem to be. So Ive started a more appropriately titled thread.In my local network I SSH (via Putty) to my Ubuntu 9.1 server and the tunnel a VNC connection through to it. No problems at all. I forward port 5900 through SSH to do this. My local servers have port 5900 blocked, so I know it is tunnelling correctly without trying to access port 5900. I cannot VNC directly to port 5900, as I would expect. Everything works just as expected.

When I SSH external to my router a I use a virtual server connection through port 22. This works just fine.I then start a VNC session, in the exact same way as I do localy, tunnelled through port 5900. Specifically I tunnel port 5900 to my.router.public.ip port 5900. However when I start up VNC it times out. The reason is that my firewall blocks a request to my routers public IP address from port 5900 on the server running SSH as follows :I have tried this both with the Ubuntu built in desktop and with x11vnc. Both give identical results. When I start x11vnc I tell it to listen on port 5900. This, as explained above, wokrs just fine in my internal network and does NOT put out any requests on port 5900.