Meanwhile.... We saw that 5.3.2 had been released via the Merchant Wave. We therefore assumed that things had been double checked and/or altered etc so we restored our dev store to 5.3.1.6 again from backups and ran the whole upgrade process again (on tip toes...
) by running the core upgrade first, then Modules, but only three at a time, so there were a few store re-deployments.... It's not a quick process doing it this way but patience etc. Anyway, this time, it's all gone okay and the dev store is now upgraded and fully functional again. We'll explore 5.3.2 and test everything over this weekend