Certainly an impressive project, John (I was thinking it made mine
look more like an Airfix kit :-)

Anyway, started on the assembly this afternoon, and have almost
resolved the fiddly bits. My friend made his own winder staircase
some time ago and recommended fixing it to the wall with 16mm
Rawlbolts - so we've got 4 of these on the long string and 3 on the
shorter one. I've left them loosely fixed for now as I'm still
shoving things around and cursing! We fixed the right angled corner
together using 2 x 75mm screws, and for now I've still got the corner
supported on bricks. Looked more closely at the mystery pieces of
wood in the kit - they actually taper so must be the wedges for the
winders. There are also two dowels for fixing the LH string to the
newel post.

I have packed around the corner area to get a good right angle and can
now get all treads to fit comfortably except the first winder, which
currently sits about 10mm too far forward on the left (in the newel
post) if it's mounted correctly in the RH string. I think my
stairwell is slightly trapezoidal and at some point not quite long
enough for the staircase (the top of the stairs rests on a joist which
I would prefer not to hack into). It's also possible that the kite is
a tiny bit too wide somewhere (or the slot in the newel post for it
not deep enough) as I can set the newel post so that the kite is
tightly fixed but the two winders are loose.

Because the bottom of the stairs is in the former under stair cupboard
which wasn't plastered I unfortunately don't have much leeway if any
dimension is too large, and a building regs compliant staircase only
fits with, according to my calculations, 6mm to spare, so I knew this
would be tight (I thought I had 6mm from the finished wall, but it
looks like the wall leans out by about 15mm, so have lost that!).

Thanks to everyone for the advice - onwards and (hopefully) upwards,
with more cursing

David.