JK has said no to Mark Evans being related to Harry Potter. In her own word, "Mark Evans is... nobody." She also gives a very interesting reson to why she chose to give Mark the surname "Evans", but for that you will have to go to JK's site. To find this news on JK's site, click the paperclips on her desk, then click the envelope that says "About the Books." The last option on the left side menu will say "FAQ Poll."
Unluckily, this means that my whole theory on Mark Evan's being the Half Blood Prince s worthless, but it was still fun to write. This does leave us with the option of Dean Tomas being the Half Blood Prince as so many of you have posted! :D
Maybe I will post my other theory later.