Ash dieback disease is a problem in Kent, and sadly some of our mature trees are affected. BWT inspect all their trees regularly, and several mature ashes have been pollarded for safety reasons. Removing diseased brances may also give them longer life expectancy.
The Ash Project has more information but so far not much is known about how the disease progresses.
If you are worried about a tree in your own garden, there is a great deal of expert information on the website of the National Woodland Trust
And here is a leaflet abour reporting disease to the Forestry Commission