I farmed on wetlands in Northumberland, fluke was endemic. There are thousands of sheep living perfectly happily all across that region, but they do need routine fluke treatment. Every other month from October thru March, and occasionally through summer too when the conditions dictate.
If you treat the fluke prophylactically you will not need to restrict their access to grass, and it is keeping sheep on other than grass that is causing the calculi. Give them ad lib grass, hay when the grass is not enough, no concentrates, no treats, and always plenty of fresh water, and drench for fluke as above.