I would do C2 in the summer, and if you dont have many exams C1 aswell for extra ums, they seem easy compared to C3 and C4 so less revision would be needed. Do chemistry and stats in january but focus on the new exams and secure good grades in those, also means you get all stats modules out of the way so you can concentrate on core maths in the summer. Unless you are applying to unis that request to know ums I wouldn't personally do the biology retake in january, if needs be do it in the summer but if you do well in the january exam it wont be necessary, even if you want an A*.