Thanks, RB79 - that's an interesting thought - fuses. Why do Skoda fit sensors that routinely fail (as you suggest)? And do they really expect customers to shell out the thick end of £600 for a whole new mirror assembly - plus extortionate labour charges of £100 an hour to fit the thing and set it up? Expletives deleted. The whole thing is maddening and depressing in equal parts. I'm happy - for that kind of money - to dip my own headlights manually, but I'll struggle to live with a rear view mirror that can't be dipped. What was wrong with the old-fashioned finger tab on the bottom edge, like all cars used to have? It was simple, it worked, and there were no crap sensors or electronics to go wrong. A classic example of smart-arse technology that only makes life worse ....
I've looked in my (comprehensive) owner manual, and they do give details of the fuses - tables listing which ones do what. But while I can find mention of the rear view mirror, I can't find any reference to the Light Assist function in the tables. So I don't know where to go from here; but your suggestion sounded promising!