Thanks @tinco, I’ll try that now. See wizardry below!
export TILLER_NAMESPACE="gitlab-managed-apps"
kubectl get secrets/tiller-secret -n "$TILLER_NAMESPACE" -o "jsonpath={.data['ca\.crt']}" | base64 -d > tiller-ca.crt
kubectl get secrets/tiller-secret -n "$TILLER_NAMESPACE" -o "jsonpath={.data['tls\.crt']}" | base64 -d > tiller.crt
kubectl get secrets/tiller-secret -n "$TILLER_NAMESPACE" -o "jsonpath={.data['tls\.key']}" | base64 -d > tiller.key
helm list --tiller-connection-timeout 30 --tls --tls-ca-cert tiller-ca.crt --tls-cert tiller.crt --tls-key tiller.key ---all --tiller-namespace gitlab-managed-apps