If it was an apple tree, I'd say it looked like woolly aphid, but that doesn't affect cherry trees. I've magnified the images, but detail isn't clear enough when magnified to see the deposits clearly. Are they weblike, or just fluffy white? Are they sticky?

Looks like mealy bug to me. The biological control would be ladybugs but it looks like you have no time to spare. Try applying one of the insecticidal soaps or oils available where you live. These guys can lay viable unfertized eggs and they can take over rapidly. Good luck. Oh, a little more information for you, these are not aphids but an unarmored scale insect and is known to be a vector of the virus known as the" little cherry disease" which is causing quite a problem in the orchards in the USA.

Mealybug isn't a problem on outdoor plants here with 4 exceptions, loosestrife -phormium, ceanothus and a couple of others, but not cherry.

I will add, though, that a foreign polyphagous tree mealybug (phenacoccus aceris) has been found on some trees in Bedfordshire, mostly on Acer and Hawthorn, and its said that can affect some fruit trees - could be that, could be scale of some sort, not sure yet.

Thanks for trying to help me. I've put on another picture, which is hopefully a bit clearer. It looks a bit like popcorn as has some brown bits to it. It is soft & a bit marshmallow like but not very sticky.

I've also just found the following link, which looks exactly like what I have, so perhaps its scale insects . Have sprayed with provado, so hopefully that will do the trick. Thanks for all your help!
http://www.thompson-morgan.com/pests/scale-insects

I have taken your photo of your leaf with pest and enlarged the image as far as my computer would take it. It is definately a scale insect.
In the states we have agricultural extension agents we can take samples to for a proper entomological identification. If you have something analogous to that I would submit a sample in order to know definately what you are dealing with. Have them advise you though as to the protocol used when submitting a sample of a possible agricultural pest.

I hope its not a fruiting cherry, Goosey, I'm assuming its only ornamental. Provado should work, if you've got the one that contains thiaproclid. I think their Ultimate Bug Spray does... might need more than one treatment, and keep a check next year, catch 'em early... but if possible, don't spray during the day when the bees are working, near dusk is a good time, and not on a windy day - and preferably not when your tree is in flower.

Loosestrife: unfortunately, by and large, there is no such facility available to amateur gardeners in the UK, other than submitting something to the RHS, if they're interested.

Yes it's just ornamental. Sill need to keep my eyes peeled for them coming back. Thank s for all your help. Are you able to help with my apple tree wilt question posted before this one please-not having much luck! Thanks again!