Hi Rachel, thanks for your response. I hadn't completed the setup int hat area, no. I have now and can confirm i still get the below error when trying to add an agent under Manage Nodes.
org.apache.commons.jelly.JellyTagException: jar:file:/var/cache/jenkins/war/WEB-INF/lib/jenkins-core-2.150.3.jar!/hudson/model/ComputerSet/_new.jelly:41:95: <st:include> No page found 'configure-entries.jelly' for class com.google.jenkins.plugins.computeengine.ComputeEngineInstance
however the Node i configured under /configure does now appear as a build agent. I will experiment more tomorrow.
Thanks for your help!