My Speedtouch 780WL has an option called NAT Loopback that allows you to access the external IP address from inside your home network. This is useful because if you run a server inside your network you can use the same IP address (and so hostname) to address the server both at home and when you are outside. To enabled NAT Loopback login to the command line (through telnet or on WIndows7 use something like terra term) and enter:
=>ip config natloopback=enabled
Then it should all work.