Evelyn have a peek at these, maybe something will take your fancy.
1. Berberis thunbergii f. atropurpurea 'Helmond Pillar' brilliant plant, deep burgundy leaves, small yellow flowers in spring and abundant berries in winter, which the birds adore. Very spiky but has an upright habit and doesn't spread too much. Low maintenance and likes a sunny or partly sunny situation.
2. Euonymus fortunei 'Silver Queen'. We have one of these in a very narrow bed against a fence. It has trained itself width ways and sort of climbs up the fence but only in its habit it's not a climber. Dense bushy habit with dark green evergreen leaves outlined with yellow in spring and white later on. Totally maintenance free. Sun or partial shade.
3. Cistus x corbariensis. Lovely little compact plant covered in an abundance of white flowers in the summer. Forms a mound of greeny-bronze leaves. Will eventually spill out over the edges of bed. We had one under a bay window in a similar depth of bed and had to take it out after about 10 years as it became too wide. Easy maintenance likes full sun.
4. Ceanothus thyrsiflorus var. repens This has deep green evergreen leaves and abundant brilliant blue flowers in late spring. We have this planted under a window and it pushes itself up the brick and has spread width ways along the wall. Again this has the potential to spread but small beds can contain it and ours has been there for 5 years + and it hangs over the path but has not outgrown its position yet. Totally maintenance free likes a sunny spot.
5. Euonymus japonica 'Benkomasaki Erecta' which has a columnar habit and is a dense evergreen of long erect stems. We have just planted one of these, found it at Wisley. I love it. It has dense columns of evergreen leaves. Brilliant structural plant.
6. There are a number of low growing hypericums, some semi evergreen. I'm desperately trying to remember the name of one we have planted that hubby calls the dinosaur plant as it has a very interesting leaf structure and habit. I'll let you know if I can find the variety.
All of the above are shrubs that are very low maintenance and can be trimmed as required. Most plants will outgrow narrow beds but can look fab overhanging, paths and gravel. If they outgrow the situ they can of course be replaced, but if you get 5-10 years out of them, that's fine.
We recently planted a narrow bed and interspersed the shrubs with 3 different varieties of fern;
Pteris cretica Albo Lineata, Asplenium scolopendrium Cristata and a Cyrtomium fortunei 'Clivicola
They look lovely, all three very different textures, colours and habits.
I sunk a rusted obelisk into one of our narrow beds and grow Clematis Fuji-Musume through it. Oh how I love it. It has blue flowers the size of your hand, is compact so perfect for obelisk growing and you get two show of flowers. The obelisk adds interest to a narrow border too.
We have a number of Dahlia Mystic Spirit which are perennial and are just growing through now. They have deep burgundy leaves and stems and an abundance of apricot flowers from July through to Oct/Nov. These are so easy, are bushy with an upright habit, don't need to be lifted I just feed them plenty of liquid feed once the foliage is growing.
Oh I'm envious, I love a gardening project. Have fun with it!! xx