I suspect that waters coming through the joints in the stones and possibly the stones themselves, there'll be a cavity tray positioned a course or so down carrying any water ingress to the facing blockwork and render.
The British climate. !! That’s just the start, in a few years that render will be discoloured especially if it’s on a weather elevation. Without a decent projection protecting the wall, concrete coping stones at the wall head, it’s just a matter of time , rain will penetrate eventually. You may have to replace them, or cap them with lead or something.