I said that I would check in Moles' book for their sowing dates.
March - May, allowing the variety to do its own thing as to harvest date.
Late sowings would probably be Zermatt (or Hannibal?) - early varieties, but this would most likely be for 'baby' leeks. And, being earlies, they would not be frost hardy.
Thanks Tim, must remember that for next year. Will be interesting to see how my late July sown leeks get on but am resting my hopes with the seedlings from Delfland Nurseries :)