I wouldn't plant them out yet in their final positions. You want them to be tall enough to just poke out of the holes you make - the deeper these are the longer the white bit of leek will be - and they need to be sturdy. Usually that means June for the broomhandle, or HH's idea, which is a good one

. You could meanwhile pot them into the extra long rootrainers, or you could prick them out into a nursery bed until they are bigger.
I see I'm the only dissenting voice