It's not impossible that some of Fletcher's men (the Londoners watch the porno film) could have known more about Carter's brother's murder, but it's unlikely too. The killers were, as far as I could determine, all Kinnear's men; basically Eric and a couple of goons.

The skinny drunken slapper tells Carter that Fletcher was at Newcastle ''on business'' two days before Jack's brother was killed, and it's obvious that the northern and southern outfits were in league professionally.
So, given that Jack's brother found out about his daughter (well, Jack's?) and the porn film racket of Kinnear's, it's likely that they did know about the murder?