Well, I fixed the problem (and an inability to upgrade W10 to version 1903 because the installed upgrade would never restart) by manually rebuilding the corrupted BCD using Bootrec.exe (and Google!). However, looking at the attachment I posted with more knowledgeable eyes now, I would have thought this solution would have been an obvious one which would have been suggested by an EasyBCD support person straight away- if one had been available! That would have saved me a fair bit of time!