Finally saw Monday's episode last night (was studiously avoiding this thread and the other until).
Can I pose a more general theory? I haven't done any reading about the producers thoughts, etc, so I might be talking out of my arse here, but:
I think we're not meant to know. We might not even know by the final episode. I think this show sees itself as not as a whodunnit, where you can see straightaway who the bad guy is, and then spend the next eight weeks honing your hate for him/her and Be Right About Your First Impressions, but more a sharp-relief example of how things can snowball when ignorance and general human-evil take over.
Jack the newsagent seemed really dodgy at first, and I'm sure a lot of people heard "underage sex" and jumped to conclusions about lots of boys, instead of the love of his life happening to be 15 years 11 months. His explanation was beautiful, and brought a tear to my eye, my partner, too (but don't tell him I told you).
David Tennant's character is one of the worst for quick-solution - laziness (for want of better word): he seems to latch on to whomever has recently angered him by withheld information that's nothing to do with anything. He didn't learn his lesson with Jack, he'd hounded vicar Rory similarly, but now is focussing on Pauline. And, considering he should have learnt a Really Big Lesson with the previous case, and doesn't have a lot of time left to redeem himself, this makes it all the more crap and tragic, IMHO.
When I saw the first episode, hell, even the first bit with the long walk down main street, my main thought was "such a perfect little community: this murder is going to lift the rock and see what's squirming underneath". And I'm definitely not disappointed on that score.