Cracks in Ceiling - Help

xleemx

Hi, I moved into a property 1 year ago and after about a month or so we noticed a few cracks in the ceiling. We just ignored them and forgot about them for a while (out of sight, out of mind, etc) but recently they caught my eye again and I noticed they had got a lot bigger. Is anyone able to help and tell me what it is, what causes it and how much I'm looking at to repair it? The house is 30 years old and these cracks are on the 2nd floor of a semi detached house with a loft above the ceiling that they're on. Any help would be greatfully appreciated, I'm very weary of calling someone and getting a cowboy builder come over. Pictures below:

EDIT - It won't let me post pictures as I only have 3 posts any idea how I can get around this to share with people who can help?