It's never bad to revise or study over the summer. I'm doing it myself, practicing exam papers and generally trying to revise subjects i'm not so good in. It can really help. Don't put too much pressure on yourself though, give yourself a break when you need one. Make yourself a timetable and follow it etc.
Nope, not worth it, I studied over the summer too, I'm more than sure I didnt get my 10 A's, Maybe about 6 or 7, This summer break between second year and third year is there as a BREAK for you for the coming year, I advise to start during october.
I agree with Patrick. The holidays are time to relax. 3rd year is a year for finishing off the little bits of course and then studying. You will have plenty of time to study during the year. October/November you should start studying for the mocks and then it should continue on until the end of the year. Plan your study and take breaks during mid-terms and bank holidays. Don't stress. It's The JC.