Hi Andrew,
I'm in S.W. France but don't go thinking we don't get it cold here. We were snowed in for 3 weeks this winter and then a massive frost in April dessimated a lot of the garden. I lost a lot of big shrubs and most of the contents of my polytunnel and had to start again. Anyway, to the matter in hand....
I leave the netting on until towards the end of August when the cabbage whites are finished with their mating. After that if I see a stray one I tend to nuke it before it finds a mate. Cruel, I know, but there you go. By then the plants are up to the top of the netting anyway and instead of just increasing the height of the poles I set them free. There's just the sprouts really by then as I would have harvested the early cabbages etc but I will have new rows of brassicas coming along by then which will be uncovered too. I'm lucky to have a lot of bamboo growing and use the new canes to make higher hoops if needed as they are bendy when just cut.
I like the square ones which are on the other thread but my hoops have worked well over the years. I would just mention that some people say you can just lay the fleece over your crops but you will find that if leaves are touching the surface the cabbage white will be able to lay her eggs on them. She'll try to get in any gap or hole too. It's a nightmare out there.... 