i am going to agree with everyone, you wont know how bad the actual damage til you strip the paint back, and also in the picture there is a round bubble section where the crack goes upward, that i am fairly confidant in saying would be a rust bubble forming and as datsinner pointed out the bog has started to lift under the paint from a fairly intensive rust patch.

as already stated you can dodgey it up and "forget" its there or you can strip that section back and refix it. If you strip it back to fix that section you will get to see just how bad the rust is/was and then start to take corrective measures from there. Highly recommending what these others have already stated. better to be safe then sorry and not wait til possibly the whole roof gives in from rust.