Leaves on saplings starting to turn black, and leaves also full of black spots.
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...to diagnose Ash die-back, don't forget that perfectly healthy Ashes are losing their leaves at this time of year.
In a diseased tree, the browning starts at the tip of the leaf and moves up the central spine.
Not the case here - the spines of your leaves are still green.
Also there would be lesions or wounds on the stems, particularly the main trunk - did you see any of those?
One of those odd cases where I sincerely hope that your ID is wrong!!
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
I did not spot any lesions on the stems,but the tree,s were only saplings, maybe 4-5ft tall.
Lat/Lng: 53.4, -2.7
OS grid ref: SJ5596