Spoiler: Part 2 What was the point
of Charles Dance having sex/developing relationship with the sister who lived in the Youlgreve house who was at least a third of his age?
It wasn't referred to again and he could have got the heroin knowledge from other circumstances. It seemed really gratuitous sex to me that was unrelated to the plot and added little.
I really didn't get it. Plus he was a vicar (and basically portrayed as a decent bloke if a bit bullying on occassion and oblivious to his serial killer daugher) and shagging a member of your congregation of that age who has come to you for help seems massively out of character.
I just don't get it.