luke i would look at the groundfloor flat if there windows are filthy explain you are doing the flat above and ask if they want theres done. if they say yes great, but if they say no... i wouldnt touch them why should they get a freebee, and they obviously arent houseproud. different story if they are nice and clean u will have to make good. I do flat above some shops, i literally sqeegee off the dripped on area...thats it.