My sets go in as soon as they arrive, usually end of March. I wait for leeks to be at least 5 inches tall and plant out April / May. After you have dibbed the hole, pop the shoot in and water as per the tried and tested method, then I cut off the top 1 to 2 inches of the plant with scissors. I find that makes a healthier and stronger leek plant.
I dont bother with onion seed anymore, I find varieties like Fen Globe and Red Baron seem to do better as heat treated sets, plus sets get a head start over seed.