I have a double-turn staircase (i.e one with a 90 degree bend at top and bottom).
There is currently a pantry under the upper end, which we'd like to get rid of in order to have a more sensible kitchen layout.

The question is - can we take away the wall under the upper newel post (because it's actually hanging from the beam under the landing bannister) or does this type of staircase need a post down to the ground?

I did do this in a previous house and it was fine, but I'm just wondering exactly how you tell. Are all staircases self-supporting so long as the top and bottom are attached?

Here's a pic of how the newel post is currently embedded in a doorway wall:

http://wookware.org/pics/online/greenbldg/staircasesupport.jpeg

I guess it won't fall down, but it might start to creak as the newel post becomes allowed to 'swing' slightly in use?

Dig in a bit, and if you find that the top newel is attached (or can be) to a trimming joist that goes all the way to wall supports at both ends (not relying on the partition), and isn't for example half cut away to fit around the newel, then it should be OK to support the newel.

If in doubt, you could work out the area of floor (incl half the width of the top half of the staircase as 'floor') that it ultimately supports, then what that's equivalent to in a joist span table (such as used to be in the Bldg Regs and you cd prob find online), hence whether your (intact) trimming joist will take it.

Will be a great simplification/space-making/improvement by the looks of it.

Here is a plan showing the ground floor walls, staircase and 1st floor joists:
http://wookware.org/pics/online/greenbldg/houseplanstair.jpeg
The cyan lines are the joists. You can hopefully see that there are two fat ones 77x182mm, one (N/S in this image) across from the outside inner leaf to the central/chimney wall, and the other(E/W in this image) from that across the edge of the landing to the next wall (actually probably to another fat joist over there, but I need to check). This is what I should apparently be calling the 'trimming joist'. That is notched into the N/S joist.

Investigations so far suggest that those chunky joists are intended to be the structural members, and the walls/doorways were done later and are not critical, but it's not totally obvious without some sums and checking how things are actually attached/supported. The hot water tank is in this area too, so that's an extra 150Kg to allow for (more if someone put a bigger one in).

The stub wall at the head of the stairs looks like it might be holding up the stairs, but it is not continuous and the fat-joist cuts it into two (slightly offset) sections: an upstairs piece and downstairs piece, as the joist goes all the way to the external wall. The plasterboard is also continuous above the wall and below the joist, suggesting that the wall was built last. Obviously right now that joist is only spanning 1.83m. If you took the wall away it would be spanning 2.75m. It will need some sums to see if that's OK or not (or if it's '1960s OK' but not '2023 OK'). It has got quite a lot of holes in for plumbing and wiring which may not quite comply with the rules in which section it should be (and if you move the support back it definitely won't. The holes are at least all central except for two top pipe notches.

The doorway in the pic, aligned with the upper newel post, probably isn't doing much, assuming the newel post is properly attached to the 2nd beam, and as fostertom says, that hasn't been choppedq through at that point. (I can't tell yet). That wall may currently be an intermediate support (at 0.85m) for the 2nd beam, 3.6m overall. It is certainly holding the newel post laterally at the moment.

And yes if it looks a bit underpowered I guess the post could be hung from the ceiling (the stairway is wider than the skeiling) (nice idea). But that ceiling has already been insulated, airtighted and decorated so making a hole for a structural support without compromising that would be somewhat inconvenient. Extra beams under the existing fat joists would be a lot easier, if less aesthetically pleasing. Or, with a lot more faff, due to wiring/plumbing, additional steel/timber could be added within the floor height.

I hope that all makes sense. I'll investigate further, do some sums, and see if I can get Tim the builder to come and have a squint.

If the walls and/or joist are supporting things then are there likely to be padding shims or extra mortar or whatever to ensure there is good contact? Or was it all built exactly to size?

I'd say getting Tim to have a look is an excellent idea. If he doesn't know the answer, he probably knows who does.

Just to check what you're proposing exactly: you're planning to take down the left most short N-S wall (not visible in your original photo)? But you say you're planning to take down the wall under the newel post, which seems like you'd need to make more changes as well.