JanG I've noticed this year over winter the Purple Sprouting Broccoli were exceptionally tall as were the Brussels Sprouts so I wondered if they were stretching for the light. Not noticed a problem in summer over the years.
Another problem is the eyelets that scaffolders use to attach it to the scaffolding can be accessed by insects as you point out and often the lines of them down the middle seem to split in strong winds.
Interesting. My PSB has been very tall lately too, but I’ve put it down to favourable growing conditions. In fact I’ve abandoned the debris netting when they’ve got to a certain height as I can’t peg it down any more, and replaced it with roughly draped black netting simply to keep the pigeons off. Could your purple sprouting and brussel spouts simply be happy and vigorous?!
I’ve not had debris netting split at the eyelet seams and, as above, I’ve noticed very little insect, including butterfly, damage through those eyelets. I’m thinking that most insects, like aphids and whitefly, will collect under any mesh etc., and the occasional incursion of butterfly eggs is reasonably easily dealt with.
Of course, debris netting won’t keep flea beetles off at the early stage of development.