My father says to start my leeks in January because that is how my grandfather sowed them (I think he grew for show). I'm reluctant to start too early because firstly, it is looking like this winter will be colder than average, so starting now might not gain much, if anything, and secondly, if they do get going, they are ready to transplant before I have somewhere prepared to transplant them. I have a couple of deep seedtrays and I sow them individually about an inch apart, then they have sufficient space to grow up until I transplant them.