The producers can put a delay - say ten seconds - on the broadcast, so the subtitle typist hears the dialogue first, and types the captions as it is broadcast 'live' to the nation.
I agree with Robinia,the subtitling unit will have followed the supplied script..The only deviation would be if a an actor got their lines at all wrong.
apparently they got afew things wrong on it. when bradley jumped and stacie ran over she shouted charlie (his real name) instead, bradley moved his hand when he should ov been dead, dot forgot her granddaughters name and max was caught putting is hand down his throat to make himself sick after bradley jumped. And i never spotted any of that because i was so caught up in it! It was put in the paper on saturday thou.