What the heck is the name of this book?

[elvish5565]

Okay there was book I read as a child and I can't remember the name of it. It was about this girl, she and her family move to this new town and buy an old gatehouse I think. She takes her horse out riding (it was a dapple grey). They ride along a fence with honey suckle, anyway she finds a gate, its rusted but with work she gets it open. She and her horse go though and walk along. They come to a house, there is a girl there wearing early period clothing. They make friends and she asks to come back again tomorrow. She has a governess and all that. So the girl goes back the next day and the next they become good friends the girl from the past teaches the other to ride side saddle. She finds a trunk in her attic that I think starts the whole thing I can't remember. They go for a carriage ride later in the book.
Does this ring any bells for anyone? I want to find this book so I can share it with my cousins. Its driving me nuts that I can remember the entire story, but not the title.