When you say that they have several houses, do they own them all? And do they rent them out? You mention that they are in a terrible state of disrepair so if they do rent them out, maybe there are some regulations that they are not meeting? I don't know a huge amount on the subject but just thinking of other avenues to explore. Also, on a different note, you can anonymously report people for benefit fraud, I think on the you gov site. We had issues with a neighbour and I ended up reporting him. I never found out what happened, but he returned sharpish to the flat so we achieved our aim.