Okay, I enjoyed the goddamn SHIT out of this movie last night. If you've not seen it stop reading and go and watch it now!
I liked how they tried to set it up so that it could either be ghosts or all explainable as real world stuff.
Except that Benigna's character was odd. I didn't get why she was at the housse in the first place at all or where she got Simon's medical records. I can't work out if he's simply meant to have got the key and read his file or if he also spoke to Benigna (if we assume there are no ghosts at all I think he would have to have).
Another aspect is what happened to the orphanage originally: the police have records showing how Tomas was killed, yet didn't go on to say how all the kids mysteriously died and their bodies were never recovered? I guess maybe 1978 was in the middle of fall of Franco so things slipped by and this is something that was understood if you were Spanish? In any case, if they were burned like that why would they then be stuffed into sacks and put back into the fireplace?
Anyway: Scary, tense movie made me jump a lot, so bottom line is: job done.