Exquisite Loft Solutions

To get a good finish its best to sugar soap the walls clean, give them a sand and a fill, then prime again white, this helps to cover any flashing from filler and gives you a brand new surface to which you can then apply the grey. if the walls are in good nick then give them a good wash and you can apply the grey straight to it.

This tends to be quite normal. i would presume the cracks that appear are dead straight and either 1200 from the wall or 1200 apart. the cracks you tend to see are usually the plasterboard lines behind the artex. if, as you say there is no push on them, and, the area is not damp, i would consider this just as movement. the floor joists on which you walk upon upstairs is also the ceiling so movement is inevitable. what i do to fix this problem is use scrim tape to tape the cracks and replaster. you can also do the same with easifill and try blend in with old artex. or just keep filling the crack, if doing this dig the crack out slightly deeper and fill.
in terms of seeing the blemish, this is called flashing. fill area, sand area, paint area, paint whole ceiling.
the chances of a ceiling falling in are very slim, unless water is involved, its more likely to just sag.