Back at the engine end I needed to do a little repair to the internal wiring before fitting the plug, which went on easily.
Having had a proper look at the cable it was obvious that it would need replacing, the copper on the neutral line was turning black and even when I trimmed it back it was no better. Time to rip it out back to the electrical panel. The new length was soon in place but actually making the connections and getting it all back together took a long time. And when it was it didn't work. Turned out it was just a fuse that I'd blown with my clumsiness. Glad to have got it done as it was getting dark, cold, windy and wet.