Well I'm delighted to be able to report that at last I've got a workable solution to this issue. Very many thanks to all that offered suggestions, much appreciated.
I eventually got a working set up using a USB WiFi adaptor. Initially I was trying with an unbranded one - the driver loading software didn't work and when I came to remove the device, the metal bit of the adaptor remained in the USB socket and had to be extracted using fine needle point pliers
The break through came when I bought a 'named brand' that worked first time. To be in control of it's IP address and to prevent it discovering a gateway off my network I've had to give it a fixed IP address outside the DHCP range of my router - this lets me leave the gateway address blank. There is however a down side in that this PC cannot join my 'Homegroup', as Windows 7 INSISTS that members must use DHCP and if you kick off the Homegroup Troubleshooter blow me if it doesn't alter it to DHCP
As the fixed IP address is outside the range controlled by DHCP on my router I cannot use Parental Control time restrictions on this PC as was suggested as the router is unaware of the machine, however not having a gateway address is I think sufficient safety in this instance.
So my 'CNC Plasma Table PC', which is physically built into the table, is now operating on two networks. 192.168.1.xxx via the WiFi adaptor which is my home network, and 192.168.10.xxx via the inbuilt network controller which is the network that the Russian Pure Logic ethernet break out board and torch height controller use.
I can now transfer files generated in the warmth of the house or my heated workshop, into the (currently) freezing cold welding shop without having to take a Thumb drive (*)
(* which I was everlastingly leaving plugged in so the PC tried to boot off it !)