He don't know Jack...!
You're quite right Kate it is a Cherry - problem is which cherry?! There are over 400 species and commonly grown cultivars and I find they are one of the more challenging groups to put a name to. There are various things that we can't see from the photo which might help, the bark, the general shape of the tree and then more detailed characters such as the hairiness of the leaves and their stalks and the pattern of toothing on the leaf margin. It clearly has biggish, pale pink single flowers which are generally not borne in big clusters and are borne just before the leaves flush, the young leaves are a pretty reddish-bronze but being unable to see some crucial characters I'm going to chicken out of giving it a definite name!
I think this is a plum. The petioles (the stems the flowers are attached to) are attached to the branch one-by-one, not in any kind of cluster. Some plums hang in clusters, but cherries do not hang individually like that. They hang in bunches, either all from the same point or leaving the main stem at differnet points. Also, the sepals (the little green or red bits that would form a star on the back of the flowers) are curled back on the older blossoms in this photo. Sweet cherries do that too (Prunus avium), as well as plums, but in general, ornamental cherries do not.
In addition to the additional information Dr Fred said would be helpful, giving the size of the blossoms helps in distinguishing one type of flower from another, particularly since it's so hard to tell that from a photograph.
wcutler is almost correct. Kate's tree is a cherry-plum (Prunus cerasifera), also known as the myrobalan. As wcutler says, the clue is the flowers being arranged one-by-one of the twig. The fruits are very similar to plums though they are not always produced. The colour of the young leaves and the faint pink colour of the flowers indicate this is P. cerasifera var. pissardii, a purple-leaved variety suppossedly originally selected by the Shah of Iran's gardener and now very common as a street tree.