Usual cause is cherry aphid infestation, can't think of anything else, unless the tree is otherwise unhealthy in some way.

It looks very healthy. If its just a few aphids I'll leave it to Mother nature to sort out. Couldn't see any on the few leaves I can reach but not many affected so far!

Well if you start seeing ants on the tree and the leaves get worse, you might need to do something.

Yes, the ants protect the aphids from their predators, for a fee of honeydew, and the aphid population could explode, unless you deal with the ants.

Maybe you have worse ants where you are Tug, but ants on plants here are simply a symptom of an infestation of scale or aphid, and once the ants are there, it becomes necessary to treat the infestation - but not the ants! They're a very useful marker that there's an issue with the plant though... and they just clear off once the infestation's dealt with anyway.

Well, it works from both ends. Without a source of honeydew, the ants go elsewhere. Without the protection of the ants, aphid predators can--but don't always--keep the infestation in check. I usually recommend dealing with the ants first, since that is usually a more localized intervention, and risks a smaller slice of the environment. If the predators work, then there are more of them to defend other plants in the neighborhood. If the predators don't work, then there are still more of them, and other organic remedies can be used, though those are harder to apply to trees.

Interesting - I'm still thinking there's something different where you are compared to where I am - in my experience, scale and particularly cherry aphid just run amok without treatment, whereas ordinary aphids might be predated by birds and ladybirds.Whatever's meant to predate them doesn't do a good job here at all, ever... which also means the ants can be left alone, of course, if you treat the infestation. Possibly we're so crowded here in the south east of UK, specially here in London suburbs, with significant pollution problems, things that predate aren't in such great numbers as the aphids. Paradoxically, pollution levels are higher in the London suburbs and south east generally than they are in central London...

It is a number of particular types of ants that "farm" aphids, but I thought that they were fairly widespread in the world. The British Isles are likely places for them to be absent, though, between high latitude and island isolation. Aphids are eaten by a number of different species: parasitoid wasps, beetles--including ladybirds--hoverfly larvae, green lacewings--if you have those in the UK--assorted true bugs, and small birds. When the nursery that I work for went organic several years ago, we had a temporary upsurge in pests, including aphids, but then things balanced out, over a couple of years. Current infestations remain small, and are soon eaten.

Well thank you both. I will look out for ants then. Not sure what I could do about a serious infestation as its a big tree? So far only a few bits are affected so I'll keep an eye on it.

Put a band of masking tape around the trunk and put a band of petroleum jelly or commercial stickum on that, being careful not to get it on the bark. On the ground, circle the trunk with ground up orange peel. Find the ants' nest, and use a spinosad bait. As I said, much easier than bringing in big spray equipment, and donning a "Moon Suit"! :)

Thank you both!