Thankyou for the suggestion. I adopted a bit of a different approach.
Since I was running the LEAP-15.6 that was previously (and still) on my Lenovo laptop (with only Grub screwed up) instead I (successfully I think) did the following:
oldcpu@lenovo:~> sudo su -
lenovo:~ # grub2-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=openSUSE_Internal --recheck
Installing for x86_64-efi platform.
Installation finished. No error reported.
and then I ran grub.cfg
lenovo:~ # grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
Found theme: /boot/grub2/themes/openSUSE/theme.txt
Found linux image: /boot/vmlinuz-6.4.0-150600.23.53-default
Found initrd image: /boot/initrd-6.4.0-150600.23.53-default
Found linux image: /boot/vmlinuz-6.4.0-150600.23.47-default
Found initrd image: /boot/initrd-6.4.0-150600.23.47-default
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
5010.132769 | DM multipath kernel driver not loaded
Found Windows Boot Manager on /dev/nvme0n1p1@/EFI/Microsoft/Boot/bootmgfw.efi
Found openSUSE Leap 15.6 on /dev/sda3
Adding boot menu entry for UEFI Firmware Settings ...
done
lenovo:~ #
I then shut down my Lenovo, unplugged the installation USB and unplugged the USB enclosure, and switched on my Lenovo.
Grub booted fine to my internal Lenovo’s LEAP-15.6 . That thankfully is back to normal from what i can see.
My next “mission” now is to check out the SSD in the USB enclosure. ie shutdown and see if I can boot to that.