- http://tobiwashere.de/2017/10/virtualbox-how-to-create-a-macos-high-sierra-vm-to-run-on-a-mac-host-system/
- https://apple.stackexchange.com/questions/307099/install-macos-high-sierra-as-virtualbox-guest-on-macos-high-sierra
Though I couldn't get a SPARSE-based image working, so I just used UDIF (read/write disk image) instead. Took a few goes however as I may not have ejected the previous sparse disks properly.
Virtualbox was pretty slow at loading and in the end it didn't find the installation target disk, so it was all for nothing - possibly because I'm using 5.1.26 instead of the 5.1.28 mentioned in the article.