They are Wooly Aphids.
Woolly Aphids get their name from the waxy excretion they carry as a form of protection from predators. At first inspection this may make them appear to be a fungus. They usually appear in the spring on some fruit trees and shrubs such as apple, pear, prunus, pyracantha, cotoneaster, elm, hawthorn and mountain ash. As with all aphids they are sucking the sugary sap, but their action causes a callous-like lump or gall to grow, which provides better access to their reward. These remain after in infestation has gone and affected branches do not usually regain their vigour so if possible they should be pruned out. If a plant is badly infested it may never recover properly and it can be best to replace it.
The adults are about 2mm long and a pinkish-brown colour, although they appear to be white with their waxy protection. They overwinter as young in crevices and cracks in the bark, emerging in the spring forming rapidly growing colonies on new shoots and at pruning cuts where the sap is more readily available. This causes weak growth of the plant and the leaves may fall due to the lack of sap to keep them functioning. They also excrete honeydew which coats the leaves and Sooty Mould may start to grow.

Each adult can produce up to five live young per day and after a few generations winged adults develop to spread to new branches and nearby trees.

Thanks very much for your very useful information Ladyessex, it was very helpful. :>))))

:o) Glad I helped.