Thanks for reading it, all.
I'm pleased to know I'm not alone, to be honest. I was beginning to worry that my own curmudgeonly and old-fashioned perspective on manners was beginning to colour my sensibilities. That it was *me* being unreasonable in expecting people to STFU.
Perhaps I should write to Cineworld after all. It's probably good to tell them why, but other than policing the aisles of the theatre for the duration of the picture (which they won't do), I can't see what they can do about a decline in manners.