wait it gets EVEN MORE CURSED
I grabbed a BS1363 (UK plug) to IEC cable. I plugged the problematic IEC-IEC cable into it. then I grabbed another known-good IEC-IEC and plugged it into that. so I have a chain of BS1363-IEC, IEC-IEC (bad), IEC-IEC (good).
I get 230V at the output.
I plug it into the old PSU.
it will not power on.
CABLE'S HAUNTED