First thing in the morning you could collect a poo sample from a few of the worst sheep ,use a spoon and a freezer bag , take this to your vet as soon as they open and get some wormer ( if you haven't already got some in stock ) then dose all sheep at the weight of the largest zw and if you bought hebs then all of them aswell at the weight of the biggest . This is your first step and even if they don't have worms it won't hurt them, allow a couple of days for the really bad ones to dry up . Your vet may advise a product to help dry them up and maybe rehydration if severe , but normally a worm dose does the job , probably best to go for group 3 or 4 wormer incase of group 1or 2 resistance . But please don't hesitate as gut damage can occur which can cause long term problems