We have an ornamental cherry which we had to prune last year and after a decent blossom we now have these leaves which look like they have some sort of disease, 90% of the tree is affected and I’ve noticed clumps of aphids on the newer leaves. What could it be?

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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Maybe give your tree a feed with a general fertiliser such as Gromore or chicken pellets to give a slow release boost to help it along