Those look like blackfly, a type of aphid. They suck the sap from the leaves, and shelter on the underside so the rain doesn't wash them off. A healthy, well -established plant will shrug off the damage, but if the tree is recently planted, or you have a very heavy infestation, they are best removed.
Sometimes spraying with soapy water gets rid of them, if not, you can use an insecticide from the GC. The downside is that insecticides are not selective, and will also kill the friendly bugs such as ladybirds and their larvae that eat the aphids.
Lots of common birds feed on aphids, especially now when they have young in the nest who need soft food. So if you put out bird feeders, the birds will help control the pests.