The answers you mark on the answer sheet are the ones they take, they are machine read so a remark isn't likely to help.
It sounds like you are using a poor system, mark the answer you think it is on the answer sheet and then if in any doubt write the question number on the working out paper to review at the end, you are only using pencil so it's eay to amend, this way you won't get the answers muddled up.