I was a minute away from disc cutting the frame in half to take it to the tip. After much huffing and puffing and sweating I can now say it's a Swiss bottom bracket. I think the bearing cups will be okay, just need a new spindle and ball bearings.
Whew, good job - I went back and forth for weeks trying to get an old bb out of an alu frame (eventually successfully) but at least there was no question as to the threading!
Standards. Who needs’em...