One thing I have been thinking about with Lucy's murder - is it all a red herring and it isn't someone on the Square? Yes, she has been living a bit recklessly, but unfortunately it's not particularly uncommon for young women to go missing or end up dead. For instance, she works in property lettings, she's effectively meeting strangers all the time. Lauren said she'd had 'three cancellations' that day for viewings, maybe one of them rang Lucy to re-arrange the viewing for that evening? Or maybe it was the person she was buying drugs off?
Also, I'm quite interested in the comments that they will find out a lot more about her now she's dead. It's got a bit of a Gone Girl flavour to it.