Thanks for the pointers - sadly I couldn't get it to connect via telnet at all - I just get (after about 1 min wait) 'Press any key to continue' followed by 'Connection to host lost'

I've done another ping, and it's still resolving the host name to an IP, but can't seem to get beyond that on port 21 (port 8080, which is the port for the router setup page) is working fine, so could it just be that the port 21 is blocked?

Is it worth me trying to set it to another port, say 7654?

Sounds like port 21 is blocked. If you can try setting up an FTP server on your lan and forwarding port 21 to that (as suggested elsewhere you may also need port 20 forwarding, but this is not always the case)

If you can get remote access to that then the issue is your router.

Otherwise the issue is your ISP blocking port 21.