Cracks in walls?

[legepe]

Hi everyone
Its a two bed first floor flat, built 1940, with horizontal cracks on the external walls both sides of the property just below the top of windows, on one side they go through the living room and into the bathroom, on the oposite side of property it is just in one of the bedrooms, the cracks are upto 4mm but only on the inside, cannot see any cracking on the outside of building
Does anyone have any idea what might be causing these cracks in walls, and are they something to be concerned about? (see attached pics)
Any advice very much appreciated
Thanks
legepe

 

[Wayners]

Window lintel. Looks worse than it's probably is.
Probably need to knock plaster off then sand and cement pushed in under lintel.
May always move a bit but visually check it and see. Lintel may have not been bedded in properly or it's just settlement