External sandstone staircase repair

[Waza74]

Anyone able to give any insight on how I would go about repairing the landing of this sandstone staircase? Also what kind of cost could I be looking at? Any feedback is greatly appreciated thanks

[Redbeard]

I have no idea about the cost, I am afraid - it depends on your local 'market' - but I would engage a structural engineer first to prepare a specification. I cannot see clearly what goes on under the stairs. Is that a door or pair of doors to an 'undercroft'? I assume that 'rusted beam' is over a door or doors. Is the space inside the doors a WC, perhaps, or just a store? Is it necessary? Could the structural support be made simpler by infilling the 'void'/store? But then what about the risk of transferring damp to the ground floor room adjacent?