Repairing ceiling cracks after a leak?

[JunglistJed]

Hey guys,

Our boiler in the top room leaked a few months ago and left 3 straight line cracks through the ceiling below and im wondering what the best method to repair them is? the obvious one is filling them with the correct filler and then sanding down but i want to avoid them cracking at a later date so if you have any tips/ a specific filler to use that would be great. We also recently had some of the ceilings replastered as its an old house and since then a lot of the ceiling where it meets the wall has cracked so it there a specific way i should fill these in?

Thanks guys!