Also, I found another minor grammatical mistake; but I've noticed that you live in the UK, and so I'm not entirely sure if it's a discrepancy between countries, but I'll point it out anyway.
You don't actually need a comma there. A comma only precedes a conjunction when there is a subject after it; for instance, this would be correct:
I do not need a comma there, but I think it's legitimised by the reading of the thing; it doesn't work as two sentences, and doesn't work well without a pause in there. My solution was to put in a comma, which, as you have pointed out, isn't needed - but improves it.
I always thought grammar was more a set of road signs than a bunch of strict rules. They direct the flow and pressure of the language, and if that bends a rule just a little, I let them do that.
Anyway, that's my little grammatical rant over. I must now return to editing Chapter Twelve and the upcoming Thirteen.