It sounds like you old walls have taken the moisture out of the new plaster, this is quite common and there's not a quick fix solution. Ideally it would be a re-plaster job, but that sounds like it could be inconvenient. Try some of the paints that claim to cover over cracks and rough plaster work if think polycell do a good one.
In this case I'd just use a good quality filler, let it dry, rub down to get a flush surface and repaint.
My question about the paint cracking is because the dreaded PVA stops the paint adhering to the plaster so as it dries it crazes into lots of little cracks. In this case the only solution is a spirit based primer over the lot before a repaint.
A mist coat is by far the best initial coat to apply on new plaster being a watered down coat of your emulsion. If using Supermatt make it 50/50 - the thinner the paint in the first instance the less water to use. Don't use Rich Matt - it don't stick to new plaster.