Hey Neil, I know this post is very old, but I'll leave this for anybody in the future who comes to this post after a Google search. There is a workaround to import KML courses into your Garmin device. There's no way to import a KML file into Garmin Connect directly, but you can import it to your device by taking the following steps:
Then once you have the route on your device, you can do the route, then save the track from that activity as a course into Garmin Connect, if you so desire.
Hope this is helpful for you and for anybody else in the future.