Answer: For such a small space, shrubs are probably more suitable than trees.
You could mix and match evergreens for their foliage with evergreens for their flowers, and deciduous, again for blooms and leaf-form. Ground-cover ivies would help hold the soil in place, and I would be tempted to plant a few clumps of spring-flowering bulbs like dwarf daffodils, multi-headed tulips, fritillaries and perhaps even snowflakes.
Eucryphia, with its lovely white flowers, makes a wonderful specimen, as does escallonia. Other flowering shrubs worth considering are skimmias, many of the viburnum family and Hebe albicans.
These, plus perhaps a hedgehog holly and a few perennials, will soon establish and should fill the bank.
If you really want a tree, then it would have to be a compact variety.