The suggestion that Cassie has dementia is intriguing but I suppose we wouldn't have many more series of Unforgotten if she's the one with dementia. Surely leaving that file in the cafe was the kind of thing any of us could do under stress? Though I do think that taking a file like that into a public place is a bit dodgy in the first place. And slightly surprised that the police still use paper files.
At the moment, all the signs are pointing to something involving James, Tim and James's son Elliott, perhaps with James and Tim covering up for something Elliott has done. (Or possibly vice versa; Elliott knows something that Tim and James don't want him to reveal.)
However, that is all slightly too obvious and we need a twist. I'm very interested in Dr Tim's character: some of the time he seems lovely and kind, but he obviously also has a very nasty streak. He is very very thoughtful and sweet to Chris, which makes me wonder whether Chris has done him a big favour in the past? Maybe Chris has covered up for him? Otherwise there is no need for someone like Tim to care about someone like Chris, who has gone from being a CEO to a homeless man with mental health problems.
About Pete. He phoned Tim to tell him that he'd seen something involving Elliott New Year's Eve. Now presumably Tim already knows this, and is worried about Pete telling the police, so perhaps indeed it is him and/or James who arranged to have him stabbed.
Keeping an open mind on the twins thing. It could be a red herring.