Sorry, need to ask you a question - is your cherry tree an ornamental, flowering Japanese variety, or a fruiting one, or at least a non Japanese variety?

Just a common British Cherry Blossom,just flowering not fruit.

The reason I ask is because Japanese cherries don't tend to suffer from Cherry Aphid, but as yours isn't Japanese, then that's most likely the problem with it. Control is by a winter tree wash in December (Vitax Winter Tree Wash), and either Provado Ultimate Bugkiller Concentrate, or Westland Plant Rescue Bug Killer Ornamental Plants in spring, or as soon as the first aphids are seen (which is usually spring). All these are to be mixed into a spray formulation, so if you can't spray the whole tree, its not going to be very effective - depends how big your tree is really. You could try spraying with either the Provado or Westland product now, but its always best to nip this trouble in the bud at an early stage.

Vitax Winter Tree Wash,if i do this in the winter with no leaves on is that it for the year?

Probably not. Carry out the winter wash in December, then, when the leaves start appearing, keep an eye on them - you may well find signs of infestation again, and that's the time to spray. Both solutions need to be applied at the right times to have any real chance of control.