Checked the leek seeds against another pack I have, and they looked the same seed. So I investigated further.
Think I've solved the problem, self inflicted I think !
I keep the wild bird seed (Sunflower) in my greenhouse, I think some may have migrated into my compost bag from spillage, or an unknown pest is storing them in there, then I used that compost for the leeks. On closer inspection, I did find some seed in the bag and some husks, they may have overpowered the leek seeds that were planted, as I did find some very stunted leek seedlings when I emptied the tray.
I've saved some of the (sunflower?) seedlings to another tray, to see what happens.
Mystery solved methinks
I'll start again