Eric Boa

As someone has pointed out, mountain ash is not related to European ash. I think this is fireblight, a bacterial disease, which is found on Rosaceae. Have a look on the RHS website. For some reason there's little mention of fireblight on Sorbus. My parents had similar symptoms on the mountain ash in their garden, which I failed to diagnose. So much for being a tree pathologist. Concentrate less on the dead leaves and the point where healthy and diseased tissue occurs. Pare away the bark and look for staining in the book. Also look for oozing from wounds.

Definitely fungal canker (Neonectria ditissima). I'd associate this with poor growth and adverse site conditions. Bacterial cankers show more pronounced swelling and production of corky tissue (which can fall away in later years and then start to resemble fungal cankers - confusing!).