open to the elements brickwall gap 30-45mm

A side neighbour has a building brickwall which buts-up close to my townhouse external-double-brick-wall (they are an a higher ground level than me, by about 1 metre) My home was built around '85, and their brickwork doesn't look too old.

I was up on the roof today, and the vertical gap between our brickwalls is about 30-43mm, running a length of about 2.7metres.
I did notice that along the rear length of our brickwalls, some kind of flat tin covering strip has been placed over this gap.
The exposed gap is toward the front half of my house.

I have noticed some white efflorescence? markings running horizontally at what would be my neighbours ground level, on the inner face of a few bricks inside the garage wall, (the wall gap is on the other side of my brick garage).
I suppose that water is pooling somewhat at the bottom of this gap, and causing the horizontal white marking on my garage wall bricks.

Is there another solution to sealing up this gap, apart from placing some kind of similar tin? strip above?
e.g. a large diameter closed-cell 'backing rod' ? (The angle of this gap is about 5 degrees, rather than flat/horizontal, at this point)