OK - because I wasn't sure if we were going to do anything inside the torch as per a111r, that is still all stripped down. I'll fit the new liner, reassemble the torch and carry out the test you have described. I'll report back asap.
You can test it without the torch. Just unscrew the orange wire from the chocbar and touch the wire to the plate that the torch cable is bolted to.
Ideally, what you want is to take the yellow wire that you put the eyelet on and connect it to the plate that the torch is connected to rather than screw B. This will make it much easier to reverse the process in the future. I think, you can probably unscrew one of the bolts holding the motor on and fit the yellow M6 eyelet then screw it back on.
If it works on the torch/motor carrier plate, in the future, all you need to do is swap the big wires over between screws A and B and not change any other wiring as has been done so far. The only problem is that with the eyelet on, the wire won't go through the gland
I carried out the tests before reassembling everything - it all works as it should. I said above that I'm such a poor welder, I wouldn't be able to tell the difference of having the correct polarity - but even I can see the benefit!!
Thank you very much for getting me to this point, I appreciate not only your knowledge but also the research you carried out on my behalf.