Subsidence beneath staircase

I am looking for reassurance but suspect that any replies may not bring good news.

The property is a 3-story Edwardian semi. Have cleared cupboard understairs in preparation for a rewire (no comments on wiring, please!). I noticed subsidence in brickwork and went under floor boards to investigate (3ft high crawling space).

Wall in question is L-shaped. It appears that structural integrity of staircase maybe compromised. To my mind, these walls support the wight of the staircase.

The gap between wall and hallway joist bearer is about 1 inch. On the adjacent wall, there is a diagonal 3/4 gap that runs though all courses of brickwork.

I suspect that problem has been in existence for many years. However, am of opinion that it would be best to get it sorted before rewire and a future buyer's survey. Are we looking at having to get wall rebuilt / underpinned?

In photos:

No.1 is location of newel post
No.2 is floor level of hallway
No.3 is hallway joist bearer
No.4 is the last full-span hall way joist

Thanks in advance
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Sally - I'm not sure you have much to worry about. Your hallway joist appears to be supported by a vertical timber, any cracking in the masonry appears to be very old as it's been whitewashed (distempered) years ago - that ziz-zag crack has whitewash in it. Many houses of this age exhibit these signs, early settlement of brickwork, cracking (heave & settlement caused by WW2 bomb damage* - very common), etc. So unless you've got new cracks or your floors (and joists) bounce then only consider cosmetic repairs. Repoint the brickwork, slate & mortar packing of gaps, and then a lick of masonry paint to tidy-up

*Even when a house didn't suffer a hit it could be damaged by the underground shockwave cause by a bomb a street away. The shockwave would often heave houses up and when it moved on the house would settle. The funny thing was that often houses close to the bomb weren't damaged structurally (apart from window & doors, and a few slates knocked off) whilst those further away would suffer.

I just love your electrics

Thank you for that - quite reassuring. Will go ahead with cosmetic repairs.

The house is in an area that was bombed during the Blitz. We also had a large crack where the outside lav was attached to main part of the house. Saw from old photos that this was braced in the mid-sixties - and 45 years later it had not moved that much more. Again the subsidence was at the corner of two adjacent walls. Apart from this all ok.