Well, I have now made a better and more permanent fixing point for the ground/earth cable. Using a 3/32 twist bit, I sank a hole roughly below the power switch.
This ensured that the cable would not be a problem to tighten down and that , in the event that the cover ever had to be removed again, there would be enough ‘play’ such that it would damage the connections on the board to which it is attached.
I have tried to ‘short out’ the USB ports since I made the change and I can’t make the computer shut down. For now, I think that this is ‘case closed’