JayD

Hi David,
Sorry to hear about your problem caused by the pesky wabbit,OK by the sounds of it you have shut the gate,with your barrier,but now you have to address the the ringbarking,otherwise the tree is domed,time is the essence,if nothing is done to the tree it will die! so your chance in sucessfully doing what I suggest has about a 40-50% chance of saving your tree,but if nothing is done death of the tree is vertually garanteed,so you could try bridge grafting, this when you cut small finger sized branches from the tree or a heathy tree of the same ,Genus species...same type,you actually bridge the gap caused by the rabbit so the tree can go about its business of survival,its worth a try,but there's a little mucking around,you have to weight it all up as to if its worth it?so if you want to give it a go I will post simple instructions and a couple of drawings to help you give this technique a go.
Maybe this will help?
JayD