Just an update. My light switch is off and when I turned the car on today in bright sunshine the instrument cluster was lit up. Went through the underground car park and I'm sure the cluster stayed lit. Will try it tomorrow with switch set to auto.
Mcmaddy: many thanks for the update and for testing out this matter for me.
I've now had the opportunity to look more carefully at the dimming curves in the admaps for the UK Golf that was sent to me and I'm 100% confident in asserting (unequivocally) that your models up there operate exactly the same as our cars down here (from a cluster illumination perspective). After all, our two countries define a "lumen" the same way and the photo transistors in VW's cars are all calibrated to respond to light in the same manner - regardless of hemisphere.
Don't bother to recheck your lighting - I'm certain that the cluster illumination will behave correctly when the switch is in the auto position.
Thanks again
Don