Thanks again, Dave.
Probably I am not making myself very clear. The behaviour I am seeing is that when I select a NEW image in Library, or Develop, and click Print, I don't see that image in the preview - I see first image that I had selected previously, and it is also that previous image that is printed. I can update the preview, and the printed file, by deselecting and reselecting "Use Selected Photos" at the bottom of the screen, but that seems unsatisfactory.
As I say, maybe I am just not getting my head into the way LR works, but it seems to me that when I am editing a photo in Develop, it is natural to click Print and be able to print that photo. (The brightness/contrast sliders thing is not a deal-breaker but it seems also that adjusting the sliders should affect the image preview.)
Sorry for taxing everyone's patience !!