Ok Jim, thanks for the info. Looking closely at the pics, I think those cracks are caused by slight movement/shrinkage, between the solid wall and the plasterboard ceiling. You say the plaster on the wall below the cracks is solid, and you know the joint is taped because you can see the scrim tape, so that's good,, what colour is the scrim by the way,, hessian coloured?
Anyway, this is what i'd do.
Gently chip away the protruding strip of plaster, give it a good brush over to remove any dust etc, then give the patch a good damp down/spray with water, even brush on a little pva if you have some. Mix up a small amount of Easifil,, bonding plaster,, or similar, then using a 3"or 4" scraper, fill in along the area and flush off, if you can use a trowel, even better. Make sure the area is still damp etc before you apply the plaster. Leave the plaster/Easifill to set, then skim over with a smooth finishing plaster or similar. Do that with each cracked area, and that'll sort the problem areas. I don't see it moving again once you've repaired it. Leave the plaster patches to dry out, give a light sandpaper if required, then you're ready to paint.