All valid points of attention but a rather sweeping statement regarding Autoglass. They are franchised, so only as good as the guys operating in you location. Mine were top notch, excellent job on replacing the Merc screen.
Regarding requesting an OEM screen, you'll almost certainly need your insurer's permission and they may well pass on some, if not all, of the extra cost. OEM for the Merc was £900. The Autoglass unit manager explained that many of the pattern screens are in fact OEM, just not branded as such. Personally I wouldn't worry about it. Autoglass won't use an OEM screen without prior written consent from the insurer.