cellar

I've just bought a house and I've discovered that the basement, which is a lot smaller than the area of the house, seems to be entirely built of hollow walls. I've also noticed parts of the ground floor which aren't over the basement flex quite a lot, considering I'd expect something like...

I live in an 1890's end terrace which has a half cellar. The wall which separates the cellar has developed a step crack, spanning about 4ft in height. It follows the mortar, although one brick has split. The joints in the cellar are a little eroded at points, and I think the crack is perhaps...