bt voyager router configuration question

Here's what I'd like to be able to do, but the BT Voyager router
software seems to be getting in the way.

I have a static IP address + BT Business Broadband. I have a web
server running on my LAN and have set up port forwarding so incoming
requests on port 80 go to the web server - that works just fine.
External users can access the web server no problem.

On a PC on my LAN I can access the web server using its local IP
address (eg 192.168.1.103), but if I try to access it via the external
IP address, I get the web page for the router. Can I configure the
router so that I can use the external IP address from within the
network, and bypass interception by the router's interface software ?