Soooo… I just pulled one of my MOTs out of the oil to try to remove the core tap and solder in a new connection. Try as I might, I could not get any solder to stick to the MOT winding. A quick sand to remove the insulation revealed why:
The winding is actually aluminium! Annoyingly, it is covered in copper coloured enamel so I didn’t notice this earlier. Oh well, you live and learn.