Thanks. I have the manual and the small wires seem ok.
My concern was the sparking when connecting the neg lead from the battery one shunt to the negative on the battery, and that there was a 13.2 voltage from that lead tested against the negative on the second battery of that bank. That seems to suggest that one of the smaller ground negative connections is in fact live and I need to try and isolate which one and why. At the moment everything is switched off so that reduces the chance of a short circuit.