Whats the best way of protecting my Brassica...

standard fence posts around the corners and a couple down each side, depending on the size. A couple  down the middle of the bed.

Put upturned plastic bottles or pot noodle pots on top, and lay scaffolding netting all over (the plastic or similar stops the netting tearing and wearing away over the long season), peg around the base - job done

I'm going to try planting climbing nasturtiums up the chain link fence that goes all the way round the veggie patch where I'm putting the brassicas. Maybe it'll keep them happy or maybe it'll just draw them in, but I guess I'm about to find out