If the dogs owners know that they have a dog with a tendancy to bite as soon as an inocent person puts a finger in a letterbox or even be aggesive as soon as they hear a person near a property, they should have a cage on the box or an outside box.
Personally I'd not have gone to a solicitor Id have gone back and carefully put a nice steak through the box with some rat poison on, in the hope that the dog would be there again and eat the meat.