You miss understood me, or I didn't make myself clear enough. I was not intending a dual boot system, just wanted to keep my old C: for as long as was necessary to convince myself that the Windows 10 install and sysprep etc went okay - which it did.
For the benefit of other readers here, this is the process I used:
Before starting, I generated a GenuineTicket.xml file using Brink's tutorial and made a full backup of my existing C: and D: drives using Acronis. I adjusted the BIOS boot settings to boot only from the new SSD and the CD/DVD drive, not the old 2TB HDD and then shut down. I disconnected the SATA cables from all the HDDs, booted up and performed Method One using a Windows 10 boot disc up to the 'Get going fast' menu, where I then switched off the PC at the power button. I connected all the SATA cables back to the HDDs, booted up, replaced the Windows 10 boot disc with my Acronis boot disc and performed another backup, this time of my new barebones Windows 10 now present on the SSD. I then swapped the Acronis disc with the Windows 10 disc and rebooted. The installation returned to the 'Get going fast' menu where I triggered Audit mode with CTRL+SHIFT+F3. Before creating the unattended answer file, I entered Disk Management from the Control Panel and reformatted the old D: partition and reassigned its ID letter back to D: (it had been changed to E:). I created the correct unattended answer file for my system, saved it and ran Sysprep. Everything went as expected, and when I finally reached my new Windows 10 Desktop, my Users folder had relocated to the D: drive on my HDD as intended. Satisfied that the process had been successful, I re-entered Disk Management and reformatted the old C: drive containing my previous Windows 7 install to erase it. Finally, I completed the Windows 10 activation using the GenuineTicket.xml file and rebooted. Windows 10 then showed up as being fully activated. Job done! Completely successful install with Users folder relocated.
I can only reiterate Kari's comments... if you read the whole tutorial and all the posts, follow the relocation process carefully and pay attention to all the details... it WILL WORK without issues. I have now done this many times over the years without problems, Win 7 to Win 10. Well done Kari!