I had a similar problem but just with AS music two years ago...I was expecting an A in performance and the external examiner marked me at a C, which brought me down to a B overall. The same happened to the others in my class - they got a few grades lower than expected - C, E and U.
My school applied to get this re-moderated and checked but they didn't change.
We all resat the performance module in year 13, and with the same examiner (!) got an A. I probably did perform a bit better, perhaps with a better choice of pieces, but it was a substantial improvement. I didn't do A2 music though.
I entered the performance module for June 2011 on my UCAS form (so entered it twice) with result pending, to let the universities know that I was resitting - but it was obvious there was something up with it compared with my other module results.
But one iffy module result shouldn't matter too much - depending on what you're applying for (I'm a medic, so music wasn't the most relevant) and it didn't have any impact on my offers - I still managed to get into Cambridge
Hope this helps somewhat - but talk to your music teachers and school about what to do, but I think the others are right and you can't apply for a remark individually. There may be variations in exam board though...