I gave up and reinstalled using Wolfman's excellent guide: Install Ubuntu-MATE using the “Something else” method and was glad to already have a separate /home partition! Even with installing some missing apps, it took a bit less than 2 hours (a fraction of what I wasted trying to fix it). Seems quite a bit snappier now too. Still probably some things to smooth out, but worth it if only for now having just EFI, ubu-Mate and home partitions. Maybe next time there's some niggling thing to fix, it will be easier. Anyway, hope this thread might be a tiny help to someone else with similar problems.