Ah.
As soon as I pop portable (embedded) python 3.12.1 into the portable vapoursynth folder, over the top of python 3.11.7 (which is the way of doing it) then things break, eg vsrepo crashes.
So, for R65 python needs to be reverted to python 3.11.7 ...
Sorry, I could not find a way to re-open the original issue. Cheers.
https://github.com/m-ab-s/media-autobuild_suite/issues/2583#issuecomment-1907081533