Based on previous series iirc we (audience) did know by this point.
The last episode reveals the rest, but don't count out a surprise there.
I'm trying to remember what the surprise was in series 1, I'm thinking it was in someone who seemed suspicious being totally innocent?
series 2 the twist was the murderers weren't charged, ethical dilemma for sunny and cassie.
Series 3 - I'm afraid it could be that the murderer is even worse than just one murder, that they've happened upon a serial killer? Who went undetected for decades? There's been real life instances like this, where serial killers have been caught due to minor parking offences etc.
I agree those knickers unlikely to be Hayley's, so who DO they belong to? The necklace also isn't one I would put with a fashionable teen at millennium, weren't chunkier necklaces more popular then? ( we need style & beauty people on that one).