I too am trying to figure out the Mansfield/Eure/royalty connection, if there is one. All of the references I have seen show Anne Eure dying before her son is born, however. Therefore either Anne is not Lancelot's wife, or the date of death is incorrect. Anne could be his mother, given the dates. I just started investigating this angle and have a lot more to do on it. If I find anything, I will let you know.