9.2 to 10.4 using VNC via the internet

I want to be able to control a Mac G4 (Mirror Doors) running OS 9.2 at work from my home Mac Mini running Tiger. I know that Tiger has a VNC client built in (I've also downloaded and installed the Apple Remote Desktop client), but as I'm thick (feel free to disagree) I'm unsure what's required when setting the thing up. I've now deduced that the Tiger VNC thing isn't an auto-detecting piece of software.

What I HAVE achieved so far is to run a server/viewer system successfully at work (on the same network) between said 9.2 machine and my Powerbook running 9.1 using AT&T "VNCServer" and "VNCViewer".

I tried to get a connection via the internet this morning using the Server software at work and a copy of VNCThing on the Mini, but the Mini can't log on to the IP address of the work Mac. I will also want to connect the other way.

Can anyone shed any light on this? Many thanks in advance if you can.