During the scene where Vimes confronts Ted Flutter with the Dark's "witness testimony," he says:
What, if anything, are we supposed to infer from that line? It's already been established that Stratford and Flutter are murderers, and their smuggling activity gets exposed just a few paragraphs later, so whatever gossip-inducing conversation they had out there must've been about something else... 
I haven't mentioned some of the other conversation between you and Mr. Stratford, because Feeney here is listening, and you should be relieved about that; gossip can be so cruel.