Science is still getting to grips with ash dieback (Chalara fraxinea); it is not known how best to try to combat it. See http://www.bbc.co.uk/news/science-environment-22869525, for instance. In this respect, it is similar to the early days of Dutch elm disease. Yes, you could collect fallen infected leaves in an attempt to try to control the release of spores, but it is difficult to know how effective you would be because: a) there would still be diseased tissue up in the trees; b) the spores are spread in the air over long distances and could reinfect that way as well. Also, the RHS advise "you should not attempt to control the disease yourself" (on one of the links below).
There are lots of things that attack ash trees, and some of the symptoms are a little similar to ash dieback
I think you are seeing one or more other infections. I see leaf rolling and thickening, which suggests moth larvae/aphids/psyllids/gall wasps/.... Also, larvae that bore inside stems (or girdle stems) can cause stem tips to die-off (turning brown) but stay on the tree; that may be happening as well.