Allotments 4 All

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.

I use debris netting and haven't noticed any problems with light levels.

I use debris netting for brassicas too. I haven’t been aware of any problems with lack of light. All kinds of brassicas develop well. I’m wondering whether you’ve noticed any problems with your brassicas which might be caused by poor light?
When butterflies have found a way in I have usually found that there’s a small hole somewhere or I hadn’t pegged it down quite well enough. I’ve heard of people sowing up the black seams, which tend to have bigger gaps, but I’ve not done that and haven’t felt the need to.
The alternatives are black butterfly netting, either the softer kind or the more obviously plastic stiffer kind. . Both are more expensive. The softer kind catch and are rather easily damaged I find, and the stiffer kind is just not as pleasant to handle. I imagine they let more light in though.
And then there’s mesh such as Enviromesh.  I don’t know how the light filtration compares but again it’s quite expensive.
Brassicas can stand a certain amount of shade so my own feeling is that debris netting works well but I’d be interested to know your experiences, G the P.

They should survive.
Not a problem for me as I haven't been able to plant any yet. Neither have most of my allotment colleagues. The weather here in Birmingham has been so wet for several months we have been unable to work the soil or plant until now.

I used to earth them up after they appeared above the soil then I earthed them by burying them again. This gave me another 2-3 weeks for them to appear again when hopefully all the late frosts were over. I sometimes think this method gave me an extra truss of potatoes with the earthing up.