It looks like European Fruit Lecanium, a soft scale insect, sometimes called the brown apricot scale. Treatment varies depending which country you're in, and you haven't said where you are.

Thank you. In the UK

Ok, well in that case, unless you fancy getting up there with a cloth moistened in methylated spirits and scraping them off with that, you need to wait till around July, when the nymphs hatch out, and spray thoroughly then with something like Provado Ultimate Bug Killer or Bug Clear Ultra. That's all very well, but a wisteria by July is usually a mass of foliage, so no easy task. I'd be inclined to try the meths treatment now, but don't get the meths on any soft foliage, and only lightly moisten the cloth with it. You'll probably need several disposable cloths...

Even doing that, if you notice the plant isn't growing well in summer, I'd be inclined to do the usual August prune earlier, in July, dispose of the cuttings, and spray what's left, spraying beneath the leaves particularly, until run off. Because this is a deciduous plant, a winter plant oil wash such as Vitax Winter Tree Wash will help, but must be done in winter, preferably on a mild day around December time.