Crack in Ceiling and Wall

[Deevant]

hi there
I have a crack that keeps reappearing regardless of what it is filled with. It is situated in my extension that was built about 5 yrs ago. It is travelling across the ceiling and down one internal wall. How do I fill it once and forall to ensure that it does not reappear days later. Across the ceiling it is hairline and spreading; however down the wall it is about 2mm.

Can anybody help? or tell me where else I can find useful info as I am new to this forum?

[welsh brickie]

its best to use a filler with flexability properties.I would try acrylic sealant it has a high flex rate and you can paint over it.You can apply it with a mastic gun.