I'm in year 13. I did music for GCSE too (OCR). I played a piece that wasn't particularly hard but from my experience, as long as you play is EXACTLY as the music is written (not just notation. Dynamics, tempo ect too), you should be able to get a good grade. I'm pretty sure they mark you based on how you play it compared to how it is written. This is what i did and I got an 8. Hope this helps.