Thanks JB, I'll give that a try.
I remember playing about with Channel Zero when I first bought the NVR and somehow - but can't remember how - I got it working (not remotely) and can't repeat it.
I'll keep playing around.
EDIT:
A bit more research and I found this:
Channel Zero RTSP live feed
That suggests that Channel Zero only works if you have a monintor (HDMI) connected directly to the NVR.
Do you have a monitor connected to the NVR in your setup?
That could possibly explain why I had working when I first bought mine and played about with it at home - I had a monitor plugged in.
In its permanent location, it is locked away in a "comms/security cupboard" so only has power and ethernet connected. Everything has to be done over the network - unless doing on site diagnostsics. That is the only time a monitor will get plugged in.
It certainly would explain why, when I tried it late last night, Channel Zero is just blank.