Found the answer in another thread started at the end of January 2019 from @Francisco-Suarez
Replace the current obsolete root CA file.
@James-Solderitsch Found it in /etc/awsiot where it needs to be named RootCA.pem. Then mosquitto needs to be restarted: /etc/init.d/mosquitto restart.
I haven't found the precise answer to my problem but I am happy I have the resources to sort it out myself now.
The resource I have found especially helpful has been a tutorial by Steve here:
"Using The Mosquitto_pub and Mosquitto_sub MQTT Client Tools- Examples"
It has also been useful to practice simple queries against a MQTT test service here:
Hope that helps someone else
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