a while ago my daughter had the walls in a bedroom skimmed and everything was fine. However, now that the walls have been painted fine cracks are appearing, seemingly more or less in the same places they were in prior to the skimming. Can anyone tell us what has gone wrong?
Difficult to say without knowing what the original walls were finished with. If it's an old property with lime plaster skimming with modern plasters is generally unsuccessful.
If it's a newer property with gypsum plaster the most likely cause is that the new plaster was applied to untreated walls and they have prematurely sucked the moisture out of the new plaster.
Not good news either way - get the plasterer back.