Finally managed to reinstall High Sierra using the
@dosdude1 patch.
For anyone having similar issues, there were two problems in my case:
1. The Patcher could not be created on the USB Stick
I had named my USB Drive as "HighSierraBoot" but I noticed that noticed that the creation process for the Patcher was unmounting this and changing the name to "OS X Base System". Presumably it would change this back to my chosen name later but while it was under that name, the process was failing. On a hunch, I reformatted the disk and called in "OS X Base System" and the process went through. I then renamed it back to "HighSierraBoot" afterwards using Disk Utility.
2. After getting the Patcher created and booting into this, the High Sierra Installer would fail with an error message saying "No packages were eligible for install. Contact the software manufacturer for assistance."
A web search showed that similar errors were linked to the system date. My system date was correct (29 Oct 2019) and knowing I had previously installed this OS, I decided to change the date to 29 Oct
2018 on a hunch and voila, the installation worked fine.
To change the date, before Step 7, "Install macOS normally onto the desired volume", in dosdude's instructions, open Terminal from the Utilities menu of the Patcher and change the system date. The date string that worked for me was "date - u 102904022018".