It isn't strong enough to stop a sheep, no. So the question is, would they challenge it? If there's plenty of good grazing and they would be and feel secure, then tame downland sheep would probably not challenge it, eh? So the next question is what would be the consequences if they did get through? If they could only get into another field, somewhere safe, then maybe risk it if you have to and it's only a short term thing. But if they would be able to get to a road, or somewhere that was unsafe for them, or somewhere where they'd be a nuisance to others, then perhaps it would be a good idea to look at an alternative or augmented fencing plan.