Search 'Scale insects' There seems to have been a lot of infestations this year.

Its said that by the time you see the white fluffy stuff the insects have gone. You can scrub the tree or paint the remaining insects with either meths or paraffin to be sure.

Sorry Steragram, but the insect is still there under the 'scale'. The life stage which has gone is the infant stage. These do move about and infest other trees.

One of mine has exactly the same problem, luckily I've already moved it well away from the other two, all its leaves have died, it happened all within the last two weeks, had to be the lovely dark one, at least I now know its not my own fault....

Not so sure that an infestation of either oyster scale or Chestnut scale (likely candidates) would kill a tree, even a small one, that quickly. They are like greenfly in that they feed on the sap of the tree. They can carry virus and infect the tree with it. However, I have seen trees almost completely smothered in scales without the tree dying, at least not quickly.
I would like to see images of these white spots to be sure of the diagnosis. They could be symptoms of something else which would kill the host plant.

Thank you for that Owdboggy. I was told years ago that years ago and have believed it ever since. I'll be more careful if I get it again.