Standards and testing: Can you retake modular (science) gcse tests even when you're in 6th form?

Yes you can, the syllabus is being phased out, P3 is only available once a year but will still be available next summer. You would need to talk to the exams officer at your 6th form and probably pay to enter (it's about �10). However, if you haven't studied Physics for a year you will probably do worse - if you are just off a higher grade you might be better off re-sitting one of the other papers sooner (i.e. in the winter) and trying to gain a few extra marks on that before you forget all your Physics.