Okay, I'll start from the begining!

Lets visualise this together! I have 5 seperate networks. All working correctly on thier own.

Network 0 - has internet connectivity. It has a DHCP service, provided by my wireless modem/router.
It's IP address, is 192.168.100.1, and it delivers IP address in the range 192.168.100.100-250
Any machine that gets an IP address from this machine, can connect to the internet, and can ping each other, and see each other

Network 1 - has a DHCP server, which is about to be retired - it has an IP address of 192.168.1.1, and it delivers IP address in the range of 192.168.1.100-250
Any macine that gets an IP address from this machine, cannot connect to the internet, and can ping each other, and see each other.

Network 2 - has a DHCP server, which is about to be retired - it has an IP address of 192.168.1.1, and it delivers IP address in the range of 192.168.1.100-250
Any macine that gets an IP address from this machine, cannot connect to the internet, and can ping each other, and see each other.

Network 3 - has a DHCP server, which is about to be retired - it has an IP address of 192.168.1.1, and it delivers IP address in the range of 192.168.1.100-250
Any macine that gets an IP address from this machine, cannot connect to the internet, and can ping each other, and see each other.

Network 4 - has a DHCP server, which is about to be retired - it has an IP address of 192.168.1.1, and it delivers IP address in the range of 192.168.1.100-250
Any macine that gets an IP address from this machine, cannot connect to the internet, and can ping each other, and see each other.

Each network is working fine. Each network now requires internet connectivity.

I have built a machine with 5 NICS in it, called junction - this machine is to connect to each existing network, and provide internet access for network 1-4

eth0 - this NIC is connected to network 0. It gets it's IP address by DHCP, and is delivered an IP address from the range 192.168.100.100-250
I can ping any host on the internet just fine.

eth1 - this NIC to connected to network 1. It has a static IP address of 192.168.10.1 It is to be used by the DHCP service running on junction to provide IP address in the range of 192.168.50.100-250 to network 1

eth2 - this NIC to connected to network 2. It has a static IP address of 192.168.20.1 It is to be used by the DHCP service running on junction to provide IP address in the range of 192.168.20.100-250 to network 2

eth3 - this NIC to connected to network 3. It has a static IP address of 192.168.30.1 It is to be used by the DHCP service running on junction to provide IP address in the range of 192.168.30.100-250 to network 3

eth4 - this NIC to connected to network 4. It has a static IP address of 192.168.40.1 It is to be used by the DHCP service running on junction to provide IP address in the range of 192.168.40.100-250