Just got off the phone after speaking to an officer of Natural Resources Wales Plant health team  http://naturalresourceswales.gov.uk/our-work/about-us/?lang=en#.UZyPcOAob9I  

The team recently put in 13 days work looking at the ash trees around Ferryside, they identified numerous examples of Ash die back disease- chalara,in managed woodlands, woodlands and surprisingly hedgerows. Their initial findings have been further confirmed by samples collected for analysis at the time of their investigation. They think that chalara has been cycling at a low level in the area for at least the last four or so years. The exact manner in which the air borne spores move around is explained in detail on a link  to the English Forestry Commission site http://www.forestry.gov.uk/forestry/infd-8udm6s

As I understand it small fungi appear on the leaf litter in June and the resultant spores are distributed by the wind. There is no particular advantage in restricting the movement of cars and people on affected areas ( i.e The whole of the Ferryside area) The good news is that we can expect to see something like 15-20% of our trees being resistant to the disease. Although a watching brief is needed on ash trees there is certainly nothing to be gained by cutting down mature ash trees unless they pose a risk to life. This story will run and run.

Denmark is often quoted when talking about chalara where up to 90% of trees were affected. Many of the trees used in Denmark were apparently a cloned variety having chosen desirable properties, obviously the unintended consequence was an inability to resist chalara. A cautionary tale ?