Thanks, yes, speaking to "all" of the local dog walkers is a great idea. Can't see the local paper running an article, but my friend who teaches at the local primary school is going to do a project with the kids (including the children of the people involved) on responsible dog walking. She's going to cover sheep & cows but also ground-nesting birds, so should benefit all of those on our land and nearby as well!
I've also made up some signs which I'll laminate and put up at the entrances to our fields.
So if they do it again there's no excuse!!