Paint Cracking: Update

From the information given, it would seem plausible that the last tenants didn't apply a mist coat or watered down pva before painting, (therefore not providing a protective barrier) - this would certainly cause the cracks you have mentioned. Personally, I would send down the ceilings to get rid of any flaking paint and to ensure the surface is smooth again. Ceilings can be sanded back to plaster with the correct sandpaper and patience. Once this has been done, a mist coat can be put into this or pva. I would then leave this a good 24 hours before applying further layers of paint. This should correct the problems with the paint on the ceiling, leaving a clean, smooth effect instead. I hope this helps Anna

Chalky residue on the ceiling could suggest a cheap contract emulsion was used. That's not really an issue as long as it stays put and doesnt flake - that might have been painted at a different time and the ceiing sealed properly. The walls however, do sound like they weren't sealed properly. My theory, assuming you have the cracks only at the top of the walls, is that they may have done a mist coat but only rolled it and didnt brush right to the edges. That would mean most paint stays ut but the edges start to lift. So that being the case you need to sand the flaking paint until you are left with a secure edge - feather the edge as much as you can. Give the walls a good wipe down to get rid of dust. Then you can apply a mist coat to the areas you have sanded back and then finish with your top coat. It's not advisable to use PVA on walls that will be painted - PVA seals the wall but prevents it from breathing and that can cause problems.

sounds exactly that, either putting neat emulsion straight over plaster or cheap emulsion like stated above

The chalky residue is caused by the paint that’s been used if it was just white Johnson’s emulsion without any vinyl purposes it will be chalky