Alive & Twitching

I have just got round to watching the Downton Abbey Christmas Special which, although corny, I found enjoyable and an improvement on much of the second series. However, there seemed to me to be an obvious flaw in the plot.

A lot of the evidence in the case against Bates, the valet, was based on remarks he had made to, or in the hearing of, various members of the Downton household. As one was given the impression that the witnesses had not previously co-operated with the prosecution, how did the latter know who to call or what questions to ask? Perhaps I missed something.