A reply to my own post
I decided to load a port scanner onto my PC and I scanned the DVR's IP address. It's quite scary how many ports are open on the DVR (good job it's behind a firewall) but anyway, I scanned from 10 to 65535 (maximum) and the port I'm after turns out to be 34600.
Rather than delete the post, I'll leave it here in case it helps someone else.