Not to sure about that mate as there are plenty of footpath's that are on legal shooting ground's and game shoots,not to mention Military Ranges.
You have to be 30 feet away from a road though.
Like i said though, i might be wrong.
iv always been told its 50 feet from the centre of a public right of way, ie road, or footpath.....
sean3513 is right there is no law that includes leads, as long as the dog is under control and will return on command the law deams this as "under control" either way they broke the law as the caused alarm and distress with a firearm....