In the 2012 Downton Abbey Christmas Special, the prosecution seems to have a very good idea of all sorts of things Mr. Bates said in various "private" moments - during a side conversation with Anna that was overheard by O'Brien, while dressing his Lordship, and while talking on the phone with his wife in a private conversation that Mrs. Hughes overheard. 

Things might have just been condensed for narrative clarity, but it really seems like the the prosecutor knew exactly who to call up and what to ask them. However, none of the other people mentioned would have voluntarily revealed anything (not even O'Brien, I think, since she seems to have had a change of heart at this point). 

I just don't see how this information could have gotten to the prosecutor, so I'm curious: how did the prosecution find out about these remarks? Did the police interrogate those three off-screen and I missed it? Or is there another character who could have overheard all those conversations, and tipped the prosecutor off?