AIU this chaps problem, the purlins cross horizontally through the area
where he wants to put velux windows. So he's considering replacing the
entire length of the purlin with 2 new steel purlins above and below
that line. However I don't understand the bit about the purlins being
braced to the walls at 2m spacings. I can't picture this at all. Are
there no roof trusses? Are the purlins supported at the gable ends
only? How come there's internal walls every 2m under the line of the
purlin?

The objective seemed to be means of escape windows (are the gable ends
not suitable?). However if there's sufficient space above the purlin
for a velux window, a built in step or steps up to the window may be
acceptable if the bottom edge of the window would otherwise be too high
for means of escape (1 meter max?).

Dave seems to be looking on basing his soultion entirely on the tables
in the approved documents. Other (timber based) solutions are possible,
but require engineers calculations to be submitted to BC.