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I've got some baby leeks in the garden which I sowed about August last year.  They are about thin pencil size at the moment. 
Can I transplant them for a crop of early leeks or should I use them as spring onions now.

They may run straight to seed

as an experiment last year i did much the same but planted straight into the ground when they were the usual size - this was just to see how late you could sow and still get a crop - so far they are a bit like you describe and if they don't look as if they are putting on useful growth I'll pull them up as I need the space - you can but try  :)