Hi zuf
I sow all my winter brassicas (except spring cauli & swedes), at the end of April because in East Yorkshire later sowings can result in insufficient growth before winter. I prefer to sow in a well prepared, sheltered seed bed on the allotment but last year moles & flea beetles caused problems. I`ve hedged my bets this year by sowing some in the ground and some in compost in 70mm x 120mm plastic cups. I never look at the dates on the packets.