I think the problem that I have found is unsurmountable at this time and I am going to look elsewhere for a solution to my immediate problem.
At this moment in time I am using a 4G connection and therefore a dynamic external IP. The IP is mapped using NoIp to a real Domain Name, but possibly things like a valid MX record do not exist.
service-discover has not created a valid .GPG containing the Carbonio Mesh Credentials. There is no file /etc/zextras/service-discover/cluster-credentials.tar.gpg
I have tried to recreate it on two separate occasions by manually resetting and then removing the JSON files and .PEM files.
But using service-discover setup LEADER_IP --first-instance --password=MESH_SECRET with the correct values does not produce a valid result. All I get at the end of the process is:
service-discover: error: unable to start service-discover.service: systemd unit startup finished with a code different than 'done'. Systemd returned: failed