If your new neighbours built a brick shed against your shared fence, came onto your property without asking, removed the two concrete panels at the bottom of the shared fence and tanked the whole thing with plastic sheeting and and mastic?
It’s the fence along our driveway, which is how they’ve got access. So the bottom of our driveway now boasts a lot of plastic sheeting on my side of the fence. A bit like a DIY damp proofing project.