Thanks again to everyone for their helpful insights - we probably will
have to leave it this weekend as there are gales forecast again and my
friend's greenhouse has today been blown sideways on the same
allotment!

I'm on an incredibly windy site. The house next door lost its
roof in 1961 and was later demolished as a dangerous structure!
When I put up my greenhouse, I didn't believe it would be strong
enough so I added cross bracing diagonally in the roof using aluminium
angle from B&Q. Its survived about four winters so far - but I always
worry when we get gales.