I cut out the toasted connectors on the regulator wiring. I checked the charging, 13.8v at 2k rpm so it seems to working OK. I suspect the connectors had a bit of corrosion causing the heat build up.
I've never known a relay to have a dead short before, but swapping it back gave continuity to earth from the fuse out point. I also found a signal wire from a coil had rubbed through.
Bad Nail, bad.
Stroppy teenager, it'll be 16 next year.