Sauté leek in butter until soft and just beginning to colour. Add vinegar and a little salt and cook over a high heat until vinegar has almost evaporated.
Bring a deep saucepan of water to a rapid boil, place a steamer on top with a plate inside and cover. Mix scallops with oil, capers and a little salt and pepper. Place leek mixture on plate in the steamer, then sit scallops on top, place apart to allow even cooking. Cover and cook until done. Small scallops will take up to 90 seconds, with ones the size of a walnut taking around 4 minutes. They are cooked when they’re just opaque inside – any more and they can be too chewy. Remove plate and serve hot, straight away, with warm, buttered bread.