Really bugs me when people are all "awwwh, hopefully they'll let you get away with it! Mistakes happen!" over crap like this - it's your responsibility to know when your exams are, and unless you had a very valid reason for being so late I really don't think they should have let you sit the exam. It's an AM exam, you sit in when everyone else does, in the morning. You were VERY fortunate that your school let your sit the paper, I guess you're just going to have to hope that the exam board are as sympathetic.