I have it working again - there must be a software bug.
It won't work until something triggers it to automatically come on - in this case I was downloading a spotify playlist. Doing so triggered the light to come on. Now works again as normal, but this isn't the first time I've had the problem.
Garmin - please fix!! Too expensive a watch for basics not to work!!
The pattern seems to be that after an activity (Pool swim, and indoor cycling) the backlight won't come on. If I use certain functions (spotify update action seems to work) they trigger the light to come on. From then on it works until I next do an activity.
