At last, my new Gerbings Junior Controller has arrived. After freezing my fingers off last week, this couldn’t come sooner. After plugging it all in, everything was working again, definitely confirming it was the old controller/wiring that was at fault.
I’ll be sending the old one back for replacement (as it has a 3-year warranty), but this will keep me warm in the meantime. A spare will be useful if I have problems again in the future.