Do you have the energy for this? if so, can you send me a 1 gram sample of whatever chemical substance you have at your disposal?

Some people would and no doubt already have let this go. I think that I personally would probably go with what they are offering and get on with my life, my rain sensors and auto lights are crap anyway and I still have the OEM one.
How much is an OEM windscreen? will their contribution of £50 make much difference to the price, I am thinking you are likely to have to dip into your pocket for anything from £100-£200 on top? If autoglass can source a VW OEM one for you? if so how much are they quoting you?
In reality by installing anything other than OEM you may potentially invalidate your cars warranty, if you can get this in writing or quote chapter and verse from your warranty you may strengthen your case.
Surely Elephant by inisisting on using non standard replacement parts are in fact invalidating their own insurance policies? My thinking is that by insisting on using non standard/ non vw approved parts then this by this increases the risk of something going wrong with your car....
Good luck, sorry reviewing my reply its of no help at all really is it. More discursive than anything else.