What are the weeds? Some weeds sheep will eat, others not. They will eat most wildflowers. The way to keep your wildflowers, is to manage the ground carefully, allowing the sheep to eat down the tougher grasses etc to allow the flowers to grow, then leaving the sheep off long enough for the flowers to seed. It definitely isn't a case of just leaving things up to the sheep.
You could harvest part of your acreage for hay, and not cut it until the flower seeds have set. This forms part of a Conservation Grazing strategy for biodiversity. There should be some information about how to go about it on The Hebridean Sheep Soc website, or at least info on who to contact, or just google the subject. Or of course someone on here is bound to be knowledgeable on the subject.