I finished Chapter 18 around lunchtime today and think it came out fairly well. This was a chapter that seemed to write itself, several times going in directions that I hadn't it considered going in. In fact, it actually ended differently than I had expected and put forward an important clue without coming across like an installment of Murder She Wrote.
Most surprisingly for me, it came out in a way that is going to mean that I can salvage great chunks of the next couple of chapters, at least in terms of the action and progress of the plot. The words you never know about until you're actually faced with the task of retyping them.
This is all encouraging, and while I doubt that I will be able to finish this draft by Halloween, I think there's some chance that I'll have it finished by some time in November. I doubt that it will be "done" done by then, but it should be good enough to shop around. Of course, it will never be completely done until some publisher forces me to stop with the revisions just so that they can get it to press.