We had the second large ash tree felled a couple of days ago. It had a lot of dead wood in the canopy, and chunks were frequently coming down. When the tree surgeon got up there he said it had the "die-back" affliction (although the tree looked reasonably healthy last year).
This is the tree before work started. A shame it's gone now, but we do have a huge pile of logs now! Some of which I have chopped up.