Mr irish spread you said and i quote....I have seen thousands and thousands of single skin walls, the vast majority of pre war walls are solid walls.
Portland cement was invented in 1824 buildings with cavitys didnt come round until the 1920's almost nearly every thing in between is built in lime what ever the size of the wall.
YOU ARE WRONG MY FRIEND
Why dont you google sand and cement on walls with no cavity like you suggested to do for some reason all you can do is find information telling you not to do it i wonder why?
Let me guess the internet is wrong im wrong the surveyors are wrong and your right