I live in a maisonette flat of a large house built circa 1900. The downstairs flat was empty for a couple of years and in much need of tlc. In the last few months new owners have been renovating down stairs. Literally all of the plaster has come off the walls and been replaced, the ceilings have been replaced etc...it has been gutted. Problem is, I am now finding, so far 5 areas of cracked plaster upstairs. Mostly superficial and can be filled painted etc, but more worrying I have discovered a diagonal crack about 3 feet or more long on an internal wall under wallpaper. This was definitely not there until recently and plaster on the other side of this wall has also cracked. I'm concerned the integrity of the wall is compromised due to all the banging etc downstairs. Any advice welcome. Thanks.