We are thinking of buying a house that was built About 40years ago but noticed there is a zig zag crack on an internal breeze block wall that goes right through to the garage. The outside of the house has no cracks should we be worried and what do we do to sort it. Thanks Karen.

That is a typical CMU zig zag settlement crack. May or may not be an issue. If it appears to have been filled in before and the crack widened, then settlement may be continuing. If the crack is fairly small and looks like it is original, it can be caulked in and repainted. Is the floor cracked? Do you know if that is a bearing wall? If it is at a gable end, probably not.

Thank you for your answer. The crack is on the ground floor in the middle of the house on a wall without a window
It looks like you could only get a sheet of paper in the gap but the same wallpaper has been there for years so I think it is original. I do not understand how there can be settlement on a house that has not had work done on it in over 30 years!

Settlement can occur no matter how old a house is. Consider the severe drought certain parts of the country had the last few years... enough moisture left the ground its caused enough contraction of the soild houses what have been there many decades as well as newer ones suffered severe foundation damage due to the settling that resulted.

Have it inspected by a certified home inspection agency (found and hired by you, not the realtor's suggestion) and/or a structural engineer. Does your state or region require disclosure of various items relating to the property by the present owner? What you can see may be only the tip of the iceberg.
Settlement can occur anytime in a structure's life for a great variety of reason, even from changes in underground (or above) water flow caused by development or other changes nearby or distant.