Prosthetics technicians were members of the crew, typically informally attached to the makeup department, often actually employed by a prosthetics subcontractor, like Millennium FX. They applied and maintained prostheses created by a prosthetics designer, like Neill Gorton, to actors. They usually reported to a prosthetics supervisor. However, depending upon the complexity of the shoot, other layers of prosthetics management -- as seen in the credits of the especially complicated Children of Earth: Day Three -- may have been involved.
Though the basic task existed in the 1963 version of Doctor Who, they've been formally credited only in DWU shows made in the 21st century.