8-10 feet is really a large shrub, but with those, you will need to consider how wide they get as well. For a small tree which reaches about 12 to 13 feet, Amelanchier Ballerina or Amelanchier Obelisk. Prunus amanogawa is known as the Flagpole Cherry - it reaches about 25 feet over time, but is never wider than about 6 feet, and has no crown as such, with all the growth being straight up into the air. Shrubs which reach this height, but also may have a width of up to 10 feet all round, are Buddleia davidii, Cornus elegantissima or Cornus spaethii, Euonymus aureum or japonica. All these can be cut to keep them within bounds, though at different times of the year for each. Cotoneaster cornubia is another treelike shrub which reaches about 25 feet if left uncut, again doesn't mind being cut to keep the spread within bounds. There are, though, many more shrubs which easily reach that sort of height.

Thanks for your useful comments - much appreciated!

The Flagpole Cherry sounds ideal as I'm after something tall and narrow. I'll check it out and (hopefully) it will be the answer to my problem.

Thanks very much once again.