Returning to the original post - the scale that I didn't recognise - I think it's clear that these:
are the same scales but have developed a characteristic 'cotton wool' excretion below them. This is
Pulvinaria regalis the 'horse chestnut scale' (although it can infest most British trees). Scales have some interesting life histories - in these pictures we have the mature females which are now dead but their scales and the 'woolly' excretion are protecting the eggs and developing young. When the young develop they will move up the tree to overwinter. The males are not scaled and are very small - they have only one brief function in life ...