Confusing crack in internal wall

[Puzzleddotcom]

I have a crack on an internal cavity wall which opens and closes in the same day! The crack lines up approximately in line with a crocodile type wall fixing which was used on the internal wall block work during an extension some 15 years ago. The external brickwork was laboriously toothed in with the original brickwork and shows no signs of distress or cracking. The crack runs from ceiling to floor but as stated can open and close completely during a day. I prefer not to paper over it, but would appreciate any helpful comments on how to stop this happening or even whether I should be concerned.
For information, the wall is South facing and both the original cavity and the "new" extension are insulated.
Thanks in advance.

 

[ban-all-sheds]

 

[tony1851]

 

[wessex101]

Sounds like it could be thermal expansion.

It is a shame that the external brickwork to the extension was toothed in, it sounds like it needs a movement joint and the rigid joint is pushing on the internal corner causing the vertical crack.

 

[Nige F]

Ask in the plastering forum - there is a type of double bead for use internal