This looked like the tree had developed a tumour on the end of a small branch. Initially I thought it was a dead branch stuck in the tree but it was actually attached!
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Forgot to say Taphrina is a fungus. The type of tree is important to know as the species of fungus is often specific to the tree.
Another possibility is insect damage to buds
Thanks, yes I think this was a beech tree.
Lat/Lng: 51.6239, -0.5836
OS grid ref: SU981926