Things are getting back to normal with KDE on mainpc and I am getting on with work today.
I was planning to have the same configuration on bedroompc (the NUC) but have run into problems with the Linux implementation of EFI when this platform is set to boot UEFI and install Debian or Ubuntu in that mode. So far with numerous tests and tweaks we seem to have hit some kind of internal limitation in the efibootmgr module for the number of times a Linux operating system can be installed on the platform (in this case, the NUC6CAYH PC with the NUC6CAYB board installed).
Since this is a fairly new NUC it is still under warranty, but since Intel doesn’t really support Linux and as I have flashed it to the latest BIOS, AND it can install and run Windows 10 without any issue, it seems it falls down onto the Debian or Ubuntu maintainers to take a look at what is going on with efibootmgr which is unable to complete the setup task of installing and configuring grub-efi-amd64. Enough of grub does get installed for the hard disk to be recognised as bootable in the NUC’s EFI bios and grub to start up to a command prompt, it seems the issue is the part of grub where it should be booting into the operating system is at fault.
Since I have tested the boot with both Debian and Kubuntu it must fall on the boot manager code and the fact of this apparent perception of a limitation to the number of installs. This computer has been successfully running other Linuxes since new, mostly Xubuntu and Debian, without problems. In fact I only recently installed the same version 9.5 of Debian with XFCE on it a few weeks ago. Given that the BIOS has been wiped and upgraded, that grub can boot to a shell, and that Win10 can run OK, it looks like I now have a process ahead of me of working with the Linux community to try and debug the problem in their EFI boot implementation.