Fell behind like 8 episodes but I got caught up over the past few nights. Still really enjoying it!
I hope Compeyson gets exposed soon though, horrible bastard.
As for Marley's killer, over the last couple of episodes I started suspecting Mr Jaggers, mainly because after the young boy (reckon he's Oliver Twist?) told Bucket that he saw a tall man with tied up hair arguing with Marley, the very next scene was a shot of Mr Jaggers and the first thing I noticed was that the length of his hair could very well be tied up in a ponytail, and after that they started showing us him having an antagonistic relationship with Bucket (and there's that one time he had a suspicious look on his face when Bucket left the room).
If not the killer, I definitely think he's at least the man who argued with Marley.
Unless I've completely missed the point and the man arguing with Marley was just supposed to be Fagin, I reckoned that was a red herring.