OP, I can only imagine how stressful this is for you, but all you can do continue on being so vigilant.
My friend bought a house 18 years ago from a probate sale. It went through and they went on a holiday, two weeks before they moved in. They came back to find the CF's at the back of their house had taken down their boundary hedge, built a wall and a put up a shed, taking 15 feet from their long garden.
They were so upset but because of a very ill parent couldn't face it.
They sent a solicitors letter with deeds showing everything but they were ignored.
6 years later the sign goes up that the CF's are selling and sale agreed.
My friend who has held the grudge, contacts her house insurance company and tells them the story.
She contacts the EA to tell them that the sale will be delayed.
Long story short the neighbours turn up all outraged at them being "unreasonable".
She tells them politely to go fxxk themselves.
The shed comes down or they pay through the nose for buying the land they illegally built on.
She was not prepared to be reasonable.
Their solicitor told them the quickest way to resolve this was to knock the wall, move the shed and put up fencing, restoring the old boundary.
It cost them serious money and stress when they were trying to move.
These CF's think they are so cute with their land grabs, but if they try to sell, when searches are done, things can be made very difficult for them.