Help! I think my plum tree is dying

Hi. We have a plum in our garden that is at least 30 years old, probably older. We have had a few problems over the last couple of years with some brown leaves and rotting fruit but generally we have always had a really good crop but the tree is really struggling this year. It has no fruit, over half of the leaves have died and the rest are brown and nearly ready to fall. I am really worried the tree is dying. Does any one have any advice on what we can do to save the tree please