Worked on this some 4 hours or so more. Instead of going at it in one marathon drawing fest, so to speak, I've been dipping in, leaving it and then dipping in again over a couple of days. That strategy seems to work better for me because I'm seeing it with fresh eyes each time ~ it's amazing how parts of the drawing jump out at me demanding to be tweaked each time I look!
I refined the foreground area a little more and, having completed the bottle itself, decided that the background did not need to go any darker. I also decided not to saturate the paper with colour, I prefer the 'grainy' look. This has been a case of 'softly, softly' pencil strokes, a constant process of adjusting colours and values across the whole image until I'm happy with it. And, guess what? I'm happy with it!