Start revising now but just do it in REALLY small steps. Like spend half an hour a week on it. You will be even more anxious about it if by the time you get to exam season, you have all these other bigger exams to worry about and you are not prepared for this resit. Just reread your post. I guess January is good to start revising for it since its not really exam season and you will still have a lot of time. Although again I would at least start reading through the content at this stage in small steps because as you get closer to exam time, your revision priorities will be towards your bigger exams.