......If boxes are being broken then you will be responsible for placing them in areas or under something to stop this happening......
I agree with ScotsGirl.
Having the bait box is one thing but you have to ensure that it is safe and nothing else can get into it whether it is inside a shed or in the open air. Most farms up here have their rodent control carried out by a company and they usually place a very large stone/breeze block on top of the box.
Don't get me wrong, the owner of the sheep has a duty of responsibility to ensure that his stock are kept in a well fenced area too.
If the other farm has a large population of rats and they aren't doing anything about controlling them, then perhaps a call to the local environmental dept may be an option.