Actually, it's very simple. You need to install Windows as GPT/UEFI (or clone it from the supplied original installation that the machine came from - that's faster). Then you have your EFI partition and, depending if you've cloned a supplied Windows, maybe a Win_RE partition (mine was called WINRE_DRV).
Then you wipe the new windows partition, but not the 'hidden' system partition/s it has created. If you have a Win_RE partition, move it to the end of the drive. You then clone your original MBR installations 'visible' drives, so not the MBR partition, just Drive c:\ and any other partitions your have. You slot them into the unallocated space between the EFI and the Win_RE partitions (if you have the latter), so in my case: SYSTEM_DRV (Fat 32) - unallocated space - WINRE_DRV (NTFS).
Then you run a Macrium Reflect Boot repair from USB (don't be perturbed by it saying it's repairing an MBR boot sector, it worked for me anyway), switch your BIOS to UEFI-first (or only, if you prefer), and you boot into your new NVMe drive. (If it doesn't work, repeat. Why doing things more than once, when you don't do anything different - which, by definition is actually a sign of madness - I don't know, but how often have I done exactly that with computers, only for it to work in the end.)
CORRECTION - SEE BELOW - sorry!
Anyway, worked for me - per se. I didn't see any performance increase, however, and found the result a little jerkier in performance (some things faster, others more hesitant), so I went back to booting from the SATA drive, which for all normal jobs is the same, by a few seconds here or there. But that's how you do it if you must. I now simply use the NVMe as a scratch drive for data-intensive projects, which is really what I wanted it for. Actually, I've hidden the c:-drive and left the boot partitions on, in case I want to return to that setup), but windows only sees the data partition, which still is about 800GB (on a 1TB drive). Hope this helps.
The Partition Sequence on the GPT drive I mentioned was wrong and is:
0 - SYSTEM_DRV (Fat 32) - 260Mb (came from the cloned manufacturer's disk)
1 - (Unformatted Primary) - 16Mb (I forgot that I HAD copied that over)
2 - C:\ (Local Disk - Windows System - NTFS Primary)
3 - D:\ (Local Disk - Data - NTFS Primary)
4 - M:\ (Local Disk - Media - NTFS Primary)
5 - WINRE_DRV (NTFS Primary) - 1GB (I just left it there, because 'why not?')
Because that copy didn't run all that well, I'll be replacing C:\ & D:\ once more and have another go. Else, I'll move my Media partition M:\ to that NVMe drive and probably replace the 2.5" SATA with an M.2 SATA card, because of heat issues, not because the M.2 will run faster.