I've got the exact same problem, I've got all A*s in all of my english exams, except one, which turned out to be a very shocking C. This brought my final grade from an A* to an A. I've got no idea what happened - it didn't seem like a hard exam.
From what I've deduced from Tullia:
I've got a UMS Mark of 25. So that means I got 27/40? That means 68%. It sounds like a C...
I just don't understand how I got that grade. Can I query it? will it cost anything? is it worth it? I'm taking Language next year.