Step like cracks in plaster. Anything to worry about?

[PhilEdT]

So I recently noticed some step like, hairline cracks in the garage this weekend. The garage is a part of the main house, so a part of it is underneath the upstaits bedroom.
The cracks are just to the internal plaster, outside garage wall looks fine. Looks like a part of the crack may have been old and previously painted over.

We're in London on clay.

Any comments appreciated, its really been one thing after the other lately!

I've attached photos of the top of the crack, mid part and the bottom.

 

[manchestermonkey]

Hi. My initial reaction is they look like thermal cracks and hence cosmetic. When any form of concrete, plaster, mortar etc. is applied/built, a chemical reaction takes place which causes the material first to expand and then contract once the material 'goes off'. As it contracts, stresses build up and the material will find its weakest point and crack.