OK, after a few hours of thrashing around, I managed to get back up and running.
I found an old app.yml file and substituted that, just deleting the old references to plugins that have since been incorporated into Discourse
This older app.yml file did not contain the code below, that I found in the later one.
after_ssl:
- replace:
filename: "/etc/runit/1.d/letsencrypt"
from: /--keylength/
to: "-d www.nzarchitecture.net.nz --keylength"
I don’t remember putting that code there myself although I had set up my site to use letsencrypt for the free security certificates, but the Set up HTTPS support with Let's Encrypt instructions don’t seem to require those lines at all, so no idea what they would have been for.
Could anything else have potentially written those lines to app.yml?. Eg. could they possibly added during an beta update?
At least for now, with those lines gone, my site is working again and up to date.
When my current ssl certificate expires I guess I may just find out what those extra lines were for.