What is the title of the episode that aired on 13 august 1981?
CREATE TABLE "series_3_spearhead_in_hong_kong" (
  "episode_num" text,
  "original_air_date_uk" text,
  "episode_title" text,
  "director" text,
  "writer" text,
  "cast" text
);
SELECT "episode_title" FROM "series_3_spearhead_in_hong_kong" WHERE "original_air_date_uk"='13 August 1981';