Does anybody know the spec (% light transmitted) of the factory fit sunset glass fitted to the rear of the Roomster?

Mine really doesn't look any different to the tinted windscreen and front side windows.

Don't know the spec but it is pretty obviously tinted -by the sounds of it your car does not have it.

Standard glass:

 

 

 

Sunset glass:

 

Pretty obvious on mine.

 

Thanks for the photos, I can see the difference in mine now, when it is sunny, when it is dull it is harder to spot, at least for me!