Cracks in plaster

Hello all. I have a 1900 terraced house the type with the extension at the rear with kitchen downstairs bathroom above.
Where the bathroom meets the main building there are cracks in the plaster.
On inspection it would appear only the plaster is cracked the brick work behind looks fine. Any ideas what has caused this? It is not progressive but has been there a long time. Should I cut out the small section of plaster and completely renew it?

 

Apologies for the photos being like they are I will try to rectify this.