Mmm....farmers not too keep on written agreements, and some can get in a huff, if you think they are not to be trusted. But, an old lady near us, rented her land out, no written agreement as far as I know. She died, and they could not get the man off, so had to sell the house with the land tenanted. Think new owner got him off somehow though.
Some of the land here was rented out for cows before we came. The farmer approached us and asked if he could still have summer grazing. We have done this, and all is well. Nothiing written down, but he had the land every sumer from 1st May until 31st October for a set price, not monthly. That is only a seasonal let, and there is no way he could become a tenant. I know with horses, the owner is supposed to take them off the land for one day a year, so as not to become a tenant, but don't think that happens nowadays.
I think if you don't know the person, its sensible to have it in writing, so you both know where you stand. Does the farmer rent full time then, not just for the summer? Personally, I would prefer the summer rental as we did, then the land is given time to rest. Unless the farmer does take his stock off and rest it, but probably not if he is paying even if not there.