Also, I bet the 76x76x12 RSJ is probably not much more than half the
127x75x13 UB in weight and cost.

The last number is the nominal weight: one is 12kg/m run,

Oh. I thought it was thickness of the base material. Are there any UB
sections less than 100mm? SuperBeam shows none, although I do like your SHS
idea as being easier to work with in terms of fabrication around the
stairway. Am I right in saying this is an advantage that applies? Is it
allowed to weld trimmer joists into place?

Would this section be better done in wood? I presume wood sections can be
kept down to 100mm depth when talking about 1m spans.

###################
| #
| #
+------------#
| #
SPL-| #
-----+------------#
#

# = load bearing wall
- = joists
| = trimmer joist
+ = weld
SPL = stair/newel post point load

Christian.