how about a nice japanese maple shrub/tree as there quit stunning and quite slim tree for its hight and not to invasive roots . some of the acer shrubs will get 15` perhaps more . your better buying your tree now and keep it in the pot untill late feb next year before the sap rises . you could put it in in autumn i guess to . i like moving them in early summer as they will grow out of the stress with plenty of watering for a good year or 2 rather than sitting over winter stressed . you may get your tree cheaper buying it in mid winter when all the leaves are of and not many people are buying garden plants .

The Japanese acers prefer a neutral to acidic soil not an alkaline one NP. Just how dry is the area you plan to plant in and is it likely to be a frost pocket being near a wall?

you can probably dig out double the amount of soil than you need for the acer and put in its preferd soil or even 3 times .water what ever you put there realy well for a good year or 2 . it is very unusual for us to dissagree generaly mg lol x .

Thanks for suggestions. The ground is not too dry and has quite good drainage. Can you think of anything else?

fatsia is quit tropical looking and an ever green . ive seen them up to about 15` and its hardy threw the winter , a plantsman im not im afraid . how about a weeping cherry or a weeping silver birch or even perhaps some membrane and a yellow bamboo with the thick canes . you could put a black veriaty next to it with the bamboo membrane .

Paulownia, Sambucus, Euonymus alatus. All have interesting bark, The Paulownia flowers before the leaves appear, If the winter isn't too hard. you could go for an unusual variety of Sambucus, Sutherlands gold is very attractive, Or the Spindle tree is lovely, has interesting seed pods and lovely autumn colour. But then looking at your question none of these grow to 20' but 15ish'. they are all imposing trees and all are beautiful in their own way.

Thank you all for your comments. I will let you know what I eventually go for.

your welcome .