I'll be kicking myself at the end of the series for not spotting clues.
At the moment we've got the missing freezer, together with semen samples. I'm hoping to get some closure on the names on Dot's list. We've got parallels between Michael Farmer and Terry (at least Terry's brief is awake) and at a stretch to the Jill Dando case.
Buckells knows Kate's undercover history, doesn't he? 
I'm not speculating yet on Jo's history but the security at the flat has to be a clue to something.I think Corbett/Clayton may have had a sister?
Lots of fridge references - Terry's freezer, KMs big pink Smeg and do we know what happened to the fridge from the brothel in s5? That one was also being used to store semen.