ref ERROR: Failed to determine the health of the cluster - #11 by Christian_Dahlqvist
Aligning some parameters in the yml files and deleting some files on the data1 node solved the issue of the nodes not creating a single cluster.
Misalignment of definitions in the yml files caused the nodes to form independent clusters.
Deleting files specified in the data1 logs (below), aligning definitions and restarting solved the issue.
"Note that the cluster UUID persists across restarts and can only be changed by deleting the contents of the node's data path [/var/lib/elasticsearch] which will also remove any data held by this node."