It seems that if I run Kibana on a coordinating node in one cluster that I am then unable to search across both the local indices in that cluster AND remote nodes
This would suggest that the only way to search across multiple clusters would be to run kibana on a completely separate elasticsearch node so that I can use a wildcard in my index pattern to read data from all clusters.
( I have to point out though that I am using version 5.6.2 as there are limitations in my configuration that may prevent me from moving to 6.x yet )