DD, who normally grows all the peas in Leicestershire, uses a technique of transplanting the leek seedlings to a deeper box, like Mum says, then waiting until the desired size (pencil thickness), then dibbing in to the final beds.
While I've never reached the size he mentions, I know that biro refill size always work as well, so fear ye not!
Our seedlings are about two inches tall, and still straggly, so a quick feed is on the cards for tomorrow (thanks Mum for the reminder)!