I've been doing some speculation about the back-stories of some of the ghosts, in the absence of news on a second series.
Kitty looks to me like she's in 1760s fashion, but I'm sure she would be less naive about life in general if she'd had 200+ years of hanging around with the other ghosts/seeing what the living occupants of the house were up to. So I really like the idea that she might have been a young girl in the 1930s attending a costume ball at Button house. Still no idea how she died!
Thomas was obviously shot in a duel, maybe by his arch-nemesis Lord Byron.
I couldn't think why the Captain would be at the house in his uniform during the war, but I think the pp suggestions that the house was used as a hospital makes a lot of sense!
The plague pit probably pre-dates the house and it was just built on top of it. They would have made the pit very deep but when the cellar was dug out, the builders probably unearthed some bodies, thought "Uh oh", and just put the flagstones down quickly and hoped nobody would notice. I know this doesn't help with why the cellar ghosts are at their burial site and not where they died!
Julian must have died of a heart attack rather than of some sort of kinky asphyxiation thing, otherwise wouldn't the orange still be in situ? 