It is Cherry blackfly. You can spray it, if it is a small tree, with a regular insecticide or use Vitax Winter Tree Wash in winter to kill off the blackfly eggs.

It's very big. And thank you, I will get on it!

My wild Cherry has the same issue this year - the mild winter and spring has acerbated the problem this year.

I have cut off infected ends over my garden but some are too high even with a ladder so will have to leave them for now. Yes it does seem the mild winter has been very good for bugs and such!