I'm not sure removing it completely is an option - although it might be preferable - I would need a stone mason to cut stone to fill the gap. I think cementing probably sounds like the way to go (and plumber's mait probably not)
I have an old cast iron waste pipe (which used to carry waste from a kitchen sink) that is no longer used and needs capping off. It is cut off quite close to the wall on the exterior of our flat and is about 2-3" in diameter.
I'm just wondering how I would go about sealing this to prevent...