Seems the service doesn’t get started until it’s actually needed.
Solution is to create a group in your config.yaml (any group) and restart HA.
I did this and restarted HA and then the service was available.
Yeah, well I’ve never needed a group, but had installed Kloggy’s very good My Garden Irrigation, which evidently makes use of the service group directives, and thereby bringing up errors when it couldn’t call the service.