Trim most of the green part off the leeks (use in the stock pot). Leave the white parts whole, slit the top and wash well under cold running water. Cook in a little boiling salted water in a covered saucepan until just tender, approximately 15 minutes.
Meanwhile put the cold milk into a saucepan with the carrot and onion, peppercorns and thyme or parsley. Bring to the boil, simmer for 5