UK here and just got it using the trick. I'm on T Mobile but bought the phone already unlocked and off contract so maybe that helps?
Anyway, using the trick, the first two interim updates were easy. I randomly disconnected the net connection and the update notification poped up. But the third - and important one - took me a good twenty minutes before I got it to pop up. The timing for the internet disconnection seemed much more crucial for some reason. I managed it at around five seconds in the end.
So for anyone who can't get the main update to appear if they have already managed the first two, persevere, because it was tricky to get right for me.