Margaret Blaine

Margaret Blaine was a senior member of MI5 in the early 21st century replaced by the Slitheen Blon Fel-Fotch Passameer-Day Slitheen. At some point prior to April 2006; Blaine was murdered by a member of the Slitheen family and her skin was used to impersonate her. (DW: Aliens of London) Still posing as Blaine, the Slitheen ran for mayor of Cardiff. (DW: Boom Town)

Behind the Scenes
Technically, Margaret Blaine never appeared in Doctor Who, only Blon Fel-Fotch Passameer-Day Slitheen masquerading as her. However the credits for Aliens of London/World War Three and Boom Town credit the character by her alias of Margaret Blaine.