The message
from James Fidell contains these words:

kenty wrote:
Can anyone recommend a tree that could be grown in a 40 cm pot.Not
acers or
orange/lemon.To add height to a sunny deck area.

A standard Bay, or perhaps a minarette-style fruit tree?

James

Bay is superb in pots. But wanting height. Thats a trickier one. Why not
a nice birch of some description. Height, pretty slender and if you get
a nice variety an awesome white bark that just begs to be touched.