1. No need to contact your insurer if you use their recommended windscreen replacement outlet. In my case this was National Windscreens and they did absolutely everything for me.
2. You can insist that you have only genuine (badged) MB parts and this also relates to your windscreen.
I went directly to National Windscreens and after following the advice of my local MB dealership, I insisted that I would only allow genuine MB parts to be used. That is our right to choose or to refuse anything other than genuine MB parts.
After asking a few questions of my insurer (NIG), the really helpful chap at National Windscreens called me back to say that this would be okay by my insurer.
The difference between a Pilkington windscreen and a genuine MB windscreen is a very small MB star symbol, printed onto the very bottom of the windscreen. Well that and the price difference. Either windscreen would have cost me £90 excess, as per my policy but if I had to buy one myself, I would have paid over £600 for one with an MB badge and under £400 for one without. Either way, both cost my insurer MORE than my premium and both are identical apart from the MB star.
Now I have a nice, shiny and new windscreen and the satisfaction of knowing that after almost 25 years of claim free and incident free motoring, that an insurer has had to pay out more than my premium.