I'm re-reading Harry Potter (all of them...because I love them!
)
Anyway, I'm really starting to think that Mark Evans is nothing more than a red herring.
In PoA. It states in the first chapter that the Dursleys are Harry's only living relatives.
Dumbeldore also says this in PS/SS. I'm pretty sure it's ben stated in CoS, GoF and OoTP.
Now, everyone knows if Ms. Rowling repeats herself, it's important. You should take her seriously. And it's been mentioned several times that the Dursleys are Harry's only living relatives, I am starting to believe that Mark Evans might not be so important.