Potentially, yes, that may have negative effects. However, it's not 100% certain that it will or that you'd even notice if it did. It depends on many other factors, so don't give up hope. You're still well within the manufacturer's spec range for Nottingham (57-70F), so any ill-effects should be reasonably minor.
If you can tell that your ferment is winding down, raising to at least 68 would probably be a good course of action at this point. You may even want to go higher, say maybe 70 or 72F. By starting at 68 and finishing at 64 the yeast may have generated some off-flavors that will hopefully get metabolized or reabsorbed by ramping the temperature up again to make sure the ferment finishes strong.
for the future: one thing to remember is that you will smell all kind of aromas coming out of the fermentor during fermentation. Those are smells that are leaving your beer, so don't make any rash decisions based on them because that's all part of the process that eventually results in (possibly delicious) beer!
