I watched the first episode of the new series and didn't have much of a clue what was going on so ended up watching the first series.
No idea why it skipped me by as it was a great first series. Quite a few times I cried, mainly with Jack and then the last series.
It wasn't obvious to me who did it, but as I knew who it was I was looking for them.
There was a part where Ellie was talking to Pauline Quirks character where she says "how could you not know it was going on under your own roof" which I think was a huge clue.