Hi Rowland,
We've tried garlic clove spray but I'm not convinced of it's worth.
However i am reasonably happy with my brassicas because of the fine 'scaffold' nets that i've fitted.
I've got good purple sprouting, savoys, Springs, kale, spin' beets all doing very nicely.
I did find one cabbage white inside a net, but since then I've tent pegged the nets very securely all round, and that's done the trick,.
(plastic pegs are £1 for ten at Wilco's) If you get replies re other sprays i too would be interested lol.