Gavinda Jaya Jaya formulated the question :
A small vertical hairline crack has appeared in the existing wall which runs
all the way to the ceiling - I'm pretty sure no lintel was installed but I
wasn't around when it was installed.

Should it have been? The interior wall is standard breeze block so there
would only be a couple of entire blocks hovering above the space.

Most certainly, there should have been a lintel installed.