I'm guessing that unlike me, most people are on holiday. So in the hope of fixing it by reference to the Repair Grub 2 from a live DVD thread, I typed this in:
sudo mount dev/nvme0n1
sudo grub-install --boot-directory=/mnt/boot /dev/nvme0n1p2
That 500M partition now has an empty EFI directory and a grub directory with 3 subdirs & 4 files. I got the "will not proceed with blocklists" error. Then I tried the same install command with the 24G ...p4 partition that now has an empty boot partition and was again treated to the blocklists error.
I'm going to read the GRUB2 & EFI recovery - Tutorial to see if I should be using EFI instead of grub.
If anyone has joined me in avoiding the holiday mania, your help and ideas will be most welcome. 