Quick udate on this. I had my windscreen replaced yesterday. In the end, I didn't go with dealership glass. It was taking too long to arrive, there was no indication of when it would be in stock and subsequent research confirmed that the non dealership glass used by Autoglass is exactly the same spec' (possibly even the same manufacturer) - it's just not branded as a Nissan windscreen.
I know that Autoglass get mixed reviews, but the two young blokes who did my car (from the Slough branch) were really good. They were on time, did the work very carefully, explained the whole process etc. The replacement seems perfect. Overall I was really chuffed with the service.