I've set up a port for access to the GUI of a 2860 but can't remember what I port number was allocated. By default access would be 192.168.82.1 but page not found. I thought I used port 8080 but page not found for that port number either.

I can, however telnet into the unit and with that ability can I discover what the port number is or perhaps even set a new one up? Any other solutions to this issue please,

You could try accessing the router using HTTPS instead, https://192.168.82.1 as that's less commonly changed.

Alternatively, try accessing the router using the telnet or SSH CLI interfaces, entering "mngt httpport ?" would show the current HTTP management port setting.

The telnet command reported that port 8080 was being used, this is what I believed it to be.

Hardwired into the same network, a positive response comes back for the ping 192.168.82.1 but access to the GUI using http://192.168.82.1:8080/ results in Page Not Found. So what is going on here then. Same of https is used.

You could say that rebooting the router would resolve this issue but I am extremely reluctant to do that as a Sure Signal device is one network device on this LAN which gives us a mobile signal when otherwise we would have nothing. It is a real bugger to get SS working and so we will have to wait for the next power cut to happen before we know that whether that simple solution is the answer.