Matt wrote:

What we did, which was a little long winded, was buy some of the long
roofing nails with the clip on caps, removed the nails and used the caps
with long screws screwed down through the ridges into the purlins. Why we
didn't use the washered self tappers as sold by Screwfix I don't know. There
was a reason but it's forgotten. I need to keep a journal :-)

Blimey! No wonder it was hard work.

--
Grunff