Steve, the Business Hub's configuration revolves (annoyingly enough) around its device list. The best way I have found around the problem you describe is as follows, and it all has to be done in the correct order for the Business Hub to build the firewall rules correctly.
1. Clear the device list on the Hub through the management console (http://gateway.2wire.net/management>troubleshooting>resets>clear local network)
(NB: This will also reset any firewall settings you have configured)
2. Set up your SBS with the x.x.x.177 IP address configured manually in the network card which connects to the Hub. (If you need DNS, an example of a BT pair is 194.74.65.68 and 194.74.65.69)
3. Confirm your router and server setup by ensuring you are now actually going out to the internet on the correct IP address by going to http://www.ipchicken.com/ or similar.
4. Now here's the fiddly part which sometimes takes some patience. Access the management console of the router again and click on troubleshooting>network tests. What you want to do on this page is set the ping number to 100 and type the IP address of your SBS in the "host" box. Hit start and wait for it to finish. Repeat this a few times. At the same time browse to a few websites from the server. The router should recognise this traffic flowing back and forth to the server and add an entry to the device list reflecting this. You can then set up the firewall rules as normal. If you still have trouble after doing the browse-and-ping-thing for 5-10 mins, leave the server switched on and connected to the router, go away, make yourself a cup of tea and watch a movie. In the couple of hours this takes the router should detect the server and add it to the device list.
5. Once you have the server showing up happily, you can configure the firewall rules as normal on the hub. Remember that you have to check the device name against the IP address to make sure you are forwarding to the correct device. This can be done either in the management console under local network>device list or in the shiny GUI under local network>advanced settings.
Good luck.
-Marc
