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When sowing brassica seeds, you need a light tilth as usual for seeds. When the brassicas have, say, 5 or 6 leaves, that is the point at which I make sure the soil is very firm around them, by heeling all round the stem.
You've already broken up the soil and loosened it, all you need to do now if you're worried is rake it over. I'm assuming you don't have solid clay, in which case you may have a bit more of a problem!