Finally finished watching this with my partner and what a superb ending (don't worry, no spoilers!). With the exception of the nonsensical notion that the main characters, despite not being policeman and worse, being witness to the crime in question, are constantly allowed to be involved in the investigation, this series was perfect...
An excellent story (I shall have to pick up a couple of Harlan Coben novels I think), incredible acting across the whole cast, a great soundtrack, well-rounded characters, superb casting (especially where different actors were required for the same characters 20 years apart). I could go on, but in short, if you haven't started this already, do! It's still available on Sky/Now TV Box Sets at the time of writing.
It's shows like this and Utopia which so many US TV shows (which seem to get all the plaudits) really should take note of. No dragging out of episodes where you wait 6 episodes just to get 1 new piece of information. No leaving cliffhangers at the end of the 1st series to leave it open for a 2nd. None of this 'oh well you've got to get through the first 2 seasons, but then it gets good, honestly'. Just a perfect one off series which is rounded off properly and if anything moves a little too quickly. Seriously, each episode contains the equivalent of around 3 'US season finale' type moments, you'll find yourself rewinding just to make sure you'd seen things right.