u/Jazzvirus

this was an arch above a window in a 600mm thick solid stone wall behind blown render. The render is removed and the is collapsed and was rendered over like this. What should be behind the arch going through there is nothing now ,and a concrete lintel on the inside.

I can rebuild the arch no problem, but as it should be a solid wall I can't think of what to do behind it. I've done loads of stonework 35+ years ago but can't remember ever doing this. This is my own house, just over 200 years old.