Thank you, hennylemon. My teacher is very keen to see the comments sheet, which I will pick up from the centre after my piano lesson this afternoon. Unless there is something very obvious, though, I probably won't challenge it. I don't want to get in to 'he said, she said' and anyway, I know that I am better than that result!
This (the bit in bold) is what is important. While it's nice to get high marks in something that you've put a lot of effort into working towards, and can be a bit disheartening when the mark isn't what you want, I think it's crucial to remember that a numerical mark is only really a measurement of how things went on that particular day, in that particular set of circumstances. It is not an absolute final say on your ability. The real reward of taking exams is the progress that is made en route to exam day and finding music you love that you might never have come across otherwise