Since writing this gist, Bytemark have published an update to use the LetsEncrypt v2 API, so an apt-get update/upgrade should work (or rather it should have already worked :)).
This is effectively a monkey-patch to overwrite existing letsencrypt functionality in Symbiosis to use the new v2 API, as the existing v1 API is being discontinued in June 2020.
This depends on a newer version of the [acme-client gem](https://github.com/unixcharles/acme-client].