What does the brick work look like from the outside - solid walls (before 1920ish) are tend to be header bonded. Post 1920, they are cavity, with stretcher bonds.
I asked because it’s difficult to see how deep that hole is. But you’re saying it’s not deep at all, you just touch brick if you put your finger in? You can’t put your finger up behind the plaster there?
Is it a solid / cavity wall? What floor is it on?
Render is really good at keeping rain out. But even better at keeping it in...