My thoughts - The boy on the canal bank is James Purefoy, this is not the guy in the taxi. In episode one there was a flash back to an explosion, I think the boy in the taxi died in that and he is some sort of relative of James Purefoy.
The white haired bloke proscecuted and due to some failing on his part, the perpatrator, possibly beardy man got off, and James Purefoy attacked him in chambers.
However 2 points 1) We have seen James Purefoy shot beardy, so I don't think he did and 2) The blood on the wall is above beardy mans head, so how that if he was shot from above as the coppers keep showing.
Then theres his mate, from whom he obviously stole his wife