Replacement glass on scratched Mark XV

Hey there everyone. This is my first post.
I bought my Mark XV from a reputable dealer here in the UK about a year ago (second hand) as a 40th birthday present. Needless to say have been chuffed to bits with it. It's had a tiny scratch from when I got it and the dealer said I could bring it in and they're replace it whenever I want. Recently it started loosing time so I took it in for a (free) service and asked them to do the glass at the same time. Was then told that they could only get non IWC glass that was 'flat' (not convex like original) - I said no way, don't mind if it's not IWC but absolutely needs to be glass and convex like the original. They've subsequently said that they can get it done but it needs to be sent off to be 'measured' then fitted by professionals etc.
So, has anyone had experience with 3rd party glass fittings (can't believe it needs to be measured - surely they know exactly the diameter required) or does anyone know of somewhere I could get original IWC glass fitted here in the UK?
Many thanks in advance.
Scott

No experience of third party glass, but why not have a correct glass fitted by sending it to the London service centre. Yes it may cost a lot but the right glass will be fitted.

Take the original crystal from IWC. Anything else is not good enough.

hmm ok thanks for the comments. I'll contact the London service centre and get a price, would definitely prefer OEM for sure.
Scott