sentences:  The story was still under consideration in April 1981.Written by Bill Lyons and also known as "The Parasites", a scene breakdown was commissioned on 22 September 1981, with the scripts commissioned on 16 February & 23 April 1982 by which point it was being considered for Season 21.Written by Lesley Elizabeth Thomas, a scene breakdown for this four-part story was commissioned on 23 April 1981.