What's the series number of the episode seen by 2.43 million viewers in the UK?
CREATE TABLE "table1_19236587_4" (
  "series_no" real,
  "season_no" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "u_k_viewers_million" text
);
SELECT "series_no" FROM "table1_19236587_4" WHERE "u_k_viewers_million"='2.43';