Geoff
As Michael says - wooly aphid. Don't pass it off as a minor pest -
remove it to save secondary infection. As Michael says canker is
common gaining access this way.

Removing them can be done with methalated spirits and a stiff brush if
you can get too them. If its on a big tree, a stong jet of water may
help to distrub them.

As they make this waxy colony, they tend to shed water making it
difficult for chemical insecticides to penetrate.

Clifford
Bawtry, Doncaster, South Yorkshire