Folks, I feel a bit uncomfortable reading some of the replies above. In the absence of any other arrangement, the duty of care for those sheep remains with the owner, and Laurelrus should not be made to feel guilty, especially after having made the effort to seek advice on here.
Yes, if I had my sheep on somebody else's land, I'd expect and want them to call me if they saw something amiss, but at the end of the day, the responsibility remains mine and mine alone.
Laurelrus - we were in much the same situation a few years ago. We had just bought our own sheep, and I told our neighbour that he would now no longer be able to use our wee bit of land for grazing (I did give him 8 months notice!). Actually what happened was that come the autumn he begged us to allow 'a few tups for a couple of weeks', but which then turned into 30 of them for six weeks! (I later found out that he had no tup-proof fields, so had come to rely on having the use of ours) These all but demolished our winter grazing and left us having to buy in far more hay than we'd planned. What's more, his sheep gave ours foot rot, pink eye, and left us with a nematodirus problem that still persists to this day. Nice eh?
If I could have my time again, I'd certainly not have let our grazing. But you live and learn, I guess.