At this age, as Twizzel says, wean them off the creep but make sure they have quality grass. Plenty of grass does not necessarily equate to to quality. Long stalky grass does not have the the feed value of succulent leafy grass as the feed value is in the leaf; not the stalks. The best grass nutritionally is what grew the night before. So if your grass is long, then strim or cut it to encourage regrowth. By cutting down on the creep you will encourage rumen development so the sheep will be able to digest forage efficiently and not need to rely on concentrates. Obviously if you were raising these to eat you might keep feeding creep to get them fat as soon as possible. But as they are the start of your flock then you want them long term to have an efficient and well developed rumen so that they can thrive on forage alone.