I appreciate all your help. I managed to run the Mac OS configuration and then MultiBeast to install the various kexts specific to my system.
I then ran the Migration Assistant to restore files from the Time Machine backup disk that I was using with the Sierra install. I haven't checked in person however the person that owns the machine has indicated that Migration Assistant not only restored their user files bit also successfully configured iMessage .
Initially I didn't restore the Applications folder as I was worried that there would be a mismatch between Sierra and High Sierra apps. However on further reading I think that Mac OS ought to be intelligent enough not to restore the Sierra Mac apps to a High Sierra system and that restoring apps will allow me to get my Microsoft Office apps working with minimal fuss. Is this correct?
I haven't yet tested audio for the new installation - this may be an area that requires some further testing and work. I'll keep you updated with regard to what I find.
One further question I have is how to finalize the USB configuration. For the Sierra install I created a specific SSDT following this guide from ReHabman.
https://www.tonymacx86.com/threads/guide-creating-a-custom-ssdt-for-usbinjectall-kext.211311/
I also used the "uia_exclude" flag in Clover to disable certain USB ports on the motherboard in order to stay below the USB port limit.
I still have the SSDT file that I created - can I (or should I?) install this file for use in High Sierra or do I need to do something different in order to finalize the USB configuration?
Thanks again for all your help!
Congrats 