Hi @craig.scott!
Thank you so much for your detailed response! I really appreciate it 
I think I understand now (I’m going to have to re-read it a couple of times and map things back to my concrete use case). Hopefully I should be able to get things working by adding a new -config.cmake file as you suggest.
I do have your excellent book yes, I’ll make sure to check out the section you recommend (I’d re-read the FetchContent section and sections after but hadn’t been able to find what I was after unfortunately 
)
Thanks again and I’ll report back when things are working, hopefully with a link to the projects (might be tomorrow now as I’m on UK time).
Much appreciated!
Tom