I've just inherited an existing site and need reconfigure some settings on the 2820. However I cant access the web config page. I can successfully telnet to the router and run commands from there. Is there any way I can identify the web port being used to access the router or alternatively enable web config if its disabled?

Grab yourself a copy of the telnet manual from the support site.

Code:

mngt httpport ?

will show the port. Replace the "?" with a port number to set it.

You may also need to check out the access list.