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Good question, shining a torch where the rsj enters the wall it looks to end at the corners, so not continuing further into the house/next door. From hardly doing any DIY, to this, it all felt terrifying but happy with my progress today now the literal dust has settled.
@Ben6899 this is my hunch as some of the bricks at the very top of the arch seem to rattle so makes me think they're not actually doing anything structural. Also there's a thin steel beam concreted underneath which you can just make out, but can't see how this arch was allowed to happen back in the day if it really was load bearing.
Well that's a bit of a result - I couldn't say for sure myself, but it would probably be helpful to know how long the RSJ extends beyond your proposed square corner.