Well as you can tell from the photo, the brassicas I had problems with are now thriving! In fact, they are doing better than the ones I planted before these ones in another spot.
I've been feeding them regularly with seaweed tonic, which I think has helped tremendously, and making sure they don't dry out.
I still have no idea why some turned up their noses and died, prehaps I needed to harden them off for longer - which I thought a week would of been fine, as they are hardy. Or what went on, but the main thing is they are happy now, and growing rapidly.
I'll take that as a win.