Thanks so much for all the advice, guys.
I learnt this the hard way. Last summer, I was silly enough to finish the entire Further course (it only took about 3 weeks and that was at a very relaxed pace) but it seriously didn't help in the end - at the very most, I was able to say "Oh, I've seen that before" and then would have to do it again. I recommend to all Further students to only complete your headstart work, but also start on your bound reference (see the relevant thread for more information).
I only plan on doing all the set headstart homework and reading my English texts these holidays. I worked myself way too hard this year and I'm going to need an adequate break.