Pretty sure this wasn't low end. 531 frame from high end builder originally. I'm coming round to my original thought that it was caused by the uneven facing on the BB shell and any damage to the shell threads can be fixed with a quick chasing out. I obviously hope so as this is by far the cheapest option and also means the frame still has years of service in it. The doubt is because the refurbisher/framebuilder is pretty well known and highly regarded.

Of course he was a little on the defensive but still convincing about ovalisation.