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I tried to post a revised ID but that doesn't seem to have worked.
If this is on lime (either Tilia platyphyllos or, more likely, T. x europaea) then this will be Eriophyes tiliae.
Thank you Peter, I guess the leaf is too decrepit (I did pick it up from the road verge!) to tell the species. If I can find the same thing on an attached leaf (I'm sure I did see that weeks ago and some may be left...) I'll try to collect some characteristics.
Lat/Lng: 51.5035, -0.738
OS grid ref: SU876790
On roadside verge