I would ask a tree surgeon as it mite be dying .

Google Ash die back......
its a fungal disease that is endangering our ash trees, forestry commission and woodland trust have more info.
I do hope its not that as sadly its usually fatal.

It won't be ash die back - Chalara fraxinus causes simple die back initially, with a dark, diamond pattern on affected branches. If you hadn't said 'large' white areas, I'd have said it was scale infestation - these produce white fluff, though I guess if there's lots concentrated in an area, from a distance, it might look like a single large area. Google "scale insects on ash tree trunk UK" for images.

Thanks Bamboo, we have quite a few ash trees and I'ma bit paranoid.......the horsechestnuts better this year, much less mite thankfully!

Possibly lichen?

You know thats a good point......some of my trees have lichen