After talking to Winston Vaz about the chainstays the verdict is that they would need both replaced to be on the safe side, which is £140. Faced with that I thought "well in for a penny, in for a pound" and have instead opted to just buy an even fancier frame to proceed with this project.
So this 653 frameset is now just sat in my garage and surplus to requirements! Anyone is welcome to collect it for a forum donation.
It needs the chainstays done at £140. I'm reasonably confident that all other tubes are fine, but Winston can confirm. Forks are definitely sound, and quite lovely. I salvaged the 653 fork decals which I can include.
If you fancied painting it yourself can take my 2-pack primer, 2 pack clearcoat and 3M respirator for £35 (I paid double that). Just buy your own pearl/metallic/base can for £5-8ish, i.e. Montana. Or ask Mario to paint it, upstairs from Winston.