Haven't read the thread, so don't know if this has already been brought up, so.......................
Latest theories on BookFace: husband and wife's sister did her in. Sister has already moved in, "to look after the children".
Apparently wife and husband have been having an affair and they killed Nicola at another location. The sister wore Nicola's clothes and hat for the dog walk, which is why no body has been found in that area - it wasn't Nicola! Dun dun dun!!
Sounds like the plot to a good movie
What's funny is that this is significantly more plausible than most of the other suggestions being thrown around