Well it wasn't as bad as I expected

After much bashing (and cursing) I concluded that the best action was
to take about 10mm off the leading inner edge of the kite, and after
that the winder box went together well. I suspect that in free space
the staircase would have gone together OK, but something was
constraining it at the bottom and I didn't have much leeway to move it
around. All I then had to do was to cut the first three risers to
size, and the winder box was completed in about 2 hours. The rest of
the staircase went together easily, and I was packing and tightening
the rawlbolts to get the right width as I went up. Here are a few
pictures of the end result, including one showing the fixing technique
I used:
http://tinyurl.com/6g9fzec
http://tinyurl.com/5vaq24c
http://tinyurl.com/63o3uuq
http://tinyurl.com/67cjf9m

All that remains to do is glue the triangular wedges to the back of
the treads (I'll leave the staircase to bed in first) and fit the top
tread into the floor of the landing. Then a load of making good!

Many thanks!

David.