The issue was the missing HST server name in the [server] url property (ie server.url in Ambari if you can find it - this was the issue - and still is - this ghosted into and then out of view in Ambari SmartSense Config tab)
Managed to change the server.url property to http://<hst server name>:9000 in Ambari (when it reappeared)
Restarted SmartSense server
And retried the SmartSense view and it now works fine.
TBC - work out why server.url property is not always visible in the SmartSense Config tabs all the time.