So much to think about, Matilda!
I'm assuming that we have now met all the key players. Whatever happened to Matthew Walsh involved the people in the first five episodes - at least one, probably more. Then there were thirty years of covering up. Some of these people would have been involved with that - maybe even people who weren't involved in the original incident. One or more of them may have witnessed or discovered the original event and used it to blackmail the presumed perpetrator(s).
We have the Fab Four plus their deceased friend.
We also have Eileen, Liz's mother. (Plus Eugenia. Plus Liz's fiancée.)
Chris Walsh, the victim's brother. (Plus the victim's son.)
Ram's wife,
Ram's brother (and parents).
Dean's wife.
Dean's brother (and mother).
Fiona's partner (and BIL).
The Hendon police trainer.
The pub landlady.
The storage facility man. (A stretch ...)
Was there anyone else?
If Cassie and Sunny didn't give Dean's address I suspect his brother managed to find it out. And as someone else suggested, I suspect the brother was the person who obliged Dean to do one more run to Calais. (I've just thought of Chris Walsh - but he didn't look as if he was profiting from drug money.) Who said it was drugs, anyway?
Really looking forward to some surprises - unexpected relationships, coincidences, misunderstandings ...