We are on the receiving end of something like this, well my mother is and I look after her affairs so me really. Smaller scale, it’s a hedge that has pushed a neighbours wall and needs repair.
Neighbour cameround with photos, fair enough we don’t know the boundaries so will look into it. No imminent danger, just a crack in his wall. Neighbour and mother have lived there 20 years.
I was getting the handyman who works on both houses to fix.
Then neighbour decided to go all legal, threatening all sorts. He s bored basically. Our house insurance covers it, plus we could get the handyman to fix. However, instructing a solicitor to peruse your neighbour is not a good way to expedite things.
So i would think pretty hard about how to move things forward directly.