I just pulled my radiator off in my living room to fit some reflective foil to increase the heat in the room and as I was stripping the old flaky wallpaper, a few chunks of plaster fell away exposing brick. Right at the very top of the exposed brick, there is a gap of about 15mm to the window sill. The gap is deep enough that I can't figure out how far back it goes.
What would be the best way of filling this gap before I pop some filler over the exposed brick? I was thinking perhaps expanding foam, but have never used the stuff so not sure this is the best option.