This seems like the best answer and should be higher.

FTR the hedge looks like a clearly established boundary, even if it isn't the legal property line. Like reading OPs post I thought it was just some scrum unimproved land between the houses but if you look at the aerials or street view (from 2012) it was clearly built as a border. How it ended up so far onto OPs property is a great question. Not sure why it wasn't built at the property line.