Because the murder will be centred around the 30th anniversary, I fully expect the culprit to be one of EastEnders' legendary or long-serving characters. That means it will either be Nick, Phil or Ian. I would have said Peter, previously, but that wouldn't work given his reaction.
I think it will be Nick, but Phil (who I think was the one burying the phone and wallet), and possibly Charlie, will have had something to so with it as well.