Heated windscreen, or maybe not?

I'm at my wits' end here - can anybody suggest what I can do?

My 2016 280L&K came with a heated windscreen as standard, which was replaced by Autoglass in October last year because of a big crack in it - I was assured that the replacement was the heated version. Over the winter I discovered that pressing the usual button had no effect on frost and ice on the screen.

The car was subsequently in at the main dealer for some unrelated work, so I asked them to check the appropriate fuse and relay were working OK - the "fully qualified" service receptionist was very reluctant, on the basis that it would cost me money, but I insisted, saying "how much can it possibly cost to check one fuse and one relay?". When I eventually got the car back, the statement was that the windscreen was not a genuine Skoda item (I had already told them that) and that it wasn't heated, because you can't see the filaments in the glass.

Even I know that these screens have a heated layer, not filaments, but I took the car to Autoglass and told them the story, whereupon they removed the plastic cover at the base of the screen, found a pair of thin wires attached to the screen which they said was for the heating, and told me to go on-line and find a replacement relay as that was definitely going to be the problem.

I then took the car to a local independent who claimed to "know these cars inside out" and who, after giving me the same guff about no filaments being visible, eventually said "the computer says the fuse and relay are OK" and that the two wires pointed at by the Autoglass people were for the automatic rain sensor and for radio reception.

Can anybody suggest what I should do, other than buy a can of aerosol de-icer?