TightVNC "Failed to connect to server"

[JSSolutions]

I have been asked by my workplace to support their systems from home. I have been supplied with a VPN (Check Point VPN-1 Secure Remote) and TightVNC.

I am able to connect the VPN, but unable to connect TightVNC to my work PC always getting a "Failed to connect to server" message when I try.

My toplogy is: -

Internet
|
| Dynamic IP
\|/
BT Router: - IP Address 192.168.x.1
| Subnet 255.255.255.0
|
\|/
NetGear Router: - Internet IP Address 192.168.x.2
| Subnet 255.255.255.0
| Gateway 192.168.x.1
| Lan side IP Address 192.168.xx.1
| Subnet 255.255.255.0
|
\|/
XP Pro: - IP Address 192.168.xx.111
Subnet 255.255.255.0
Gateway 192.168.xx.1

When I turn off the Firewall on the BT Router TightVNC works.

With the BT Router Firewall on and Ports TCP 5500/5800/5900 and UDP 5500/5800/5900 opened TightVNC doesn't work.

I have read dozens of articles on Port Forwarding/Port Triggering and Virtual Servers and have attempted dozens of different combinations of these.

What I don't understand is why turning off the BT Router Firewall allows TightVNC to work, don't I still need some sort of Port Forwarding configured?

Also I don't understand what IP Address, and on which Router, to configure the Port Forward of 5900 . Should it be: -
BT Router 5900 to 192.168.x.1
BT Router 5900 to 192.168.x.2
Netgear Router to 192.168.x.1
Netgear Router to 192.168.x.2
Netgear Router to 192.168.xx.1
Netgear Router to 192.168.xx.111
something else?

Or should the Port Forwarding be to the IP Address of my work machine which starts 172? and if so on which Router?

I am completely new to Firewalls and VNC and would appreciate any help.

Many thanks