Good idea amanda. When you go in, say something like, "I've turned up early as I just couldn't suffer the pain any longer" let them know how much discomfort you have been in!
If the receptionist is the same one all the time then all the better, as she won't be able to wriggle out by saying it wasn't her etc. To hell with confrontation, you are in pain are you not? Make sure you point out that you feel neglected by the practice, that they failed in their duty of care. Easier said than done, but be calm!
Make sure you let them finish the treatment, I can understand you don't want them too, but if you have it done elsewhere you may be charged privately.