For a fascinating article on this moth and caterpillar click on the picture.
You may also have grey aphid - usually spotted only when you are on a caterpillar hunt. Aphid control is always a problem but one of the newer suitable pesticides is Provado ultimate bug killer, which is suitable for use on brassicas.
You should not use soap-based contact insecticides on brassicas as they remove the essential waxy coating on the leaves and the plant is often killed or severely damaged.