Why dibble-in leeks?

Yes Plottered, that's OK too. If they can survive planting out at smaller than pencil sized up here and grow big, strong and tall, then you'll be fine.

"My sister Dolly has a nice shiny  silver propelling pencil. She was given it by our Great Aunt Violet, for keeping an eye on the door while she entertained the gardener...."

"Will that do Captain Mainwaring..."?

I got mine to about 5mm stem thickness and i'm halfway planting them out. I fed once a week with liquid seaweed and they look very nice! I dib a 6" hole with a spade handle and space them 8" or so apart. I don't trim the tops and don't snip the roots either, as Growster says, just wet the roots thoroughly and they plop down the holes lovely! I have 60 planted and about the same again to go!