I think it could be good to use the first week at least to take a step back and look at your situation. How are you going so far? Why haven't things worked out as well as you'd like? What have you done well, and what have you done poorly? What can you change? What goals are you setting? Are they realistic? How are you going to make sure that the change actually happens and sticks? Who can you get to help you and how can you keep up the motivation a bit?
I reckon you should also pull together Unit 3 - e.g. organising your notes, testing yourself lightly across all areas to see where you have any big gaps, going over what you did well and what you did poorly at, and figuring out what you're going to have to keep studying throughout the year. I wouldn't call this 'studying for exams', so much as collecting yourself together after Unit 3 so everything's not a complete mess by the time you get to the exam and have completely forgotten it all anyway.
Then you can do the homework and study for your SACs.
But, you also absolutely need a holiday! Take a bit of time out to do activities you enjoy, to take a whole-day trip hiking somewhere you love, to just get some sleep and relaxation time, to look at dealing with any personal issues you have. Try and de-stress so you can come back fresh for Term 3, possibly the killer term. Don't even touch work on weekends, even every weekday, and even if you do work 5 days a week, don't pull 8-hour days.
All the best
