Heidi Stafford and her husband Jay were members of the Slitheen family who posed as humans. Bloorm Vungah Bart Gossimar-Day Slitheen posed as Heidi. She claimed to be the real mother of Luke Smith, who they said was named Ashley Stafford. (TV: The Lost Boy)
It turned out that Heidi and Jay Stafford never existed. They were invented by Bloorm Vungah Bart Gossimar Day Slitheen, Dax Fex Fize Gossimar-Day Slitheen and Mr Smith. Bloorm Vungah Bart and Dax Fex Fize had killed June and Marco Goss and used these human skins to pose as Heidi and Jay Stafford to steal Luke Smith's Telekenetic energy. (PROSE: The Lost Boy)