You’ve got a flat, felt porch that’s right up against the brick wall, so it could be that something’s failed along the flashing. Also it looks like you could do with some re-pointing around that area:

The only thing that’s bugging me is that the water appears to have come in all of a sudden - I would have thought that you’d start noticing minor damage before a massive leak.
Did the upstairs neighbours double-check that it was nothing to do with them?