Mismatched kubeadm versions & kubeadm join

When joining a cluster with versions of kubeadm and/ or kubelet which don't match the master, you get an error like:

[kubelet] Downloading configuration for the kubelet from the "kubelet-config-1.11" ConfigMap in the kube-system namespace
configmaps "kubelet-config-1.11" is forbidden: User "system:bootstrap:tn8mhk" cannot get configmaps in the namespace "kube-system"

This can be fixed by installing matching versions (e.g. apt-cache madison kubeadm then apt-get install kubeadm=OUTPUT_FROM_APT-CACHE

Use apt-cache policy PACKAGE_NAME to get the currently installed version on the master.