Sorry bad typo - The BT Hub I meant is the BT Router - there is no hub. The current setup is internet line comes into VDSL Modem which then connects to WAN port on BT Router which then connects to the Switch and then on to all the other computers.

As I said I am changing this so that it will be modem-BT Router - Linux Gateway - Switch - Computers. I did not do this originally because when I was setting up the Linux Gateway I could not get it to work with two nics.