Have you tried using the Plant Finder facility on the RHS website?
Filling a small garden with small plants seems logical but actually just makes it look smaller and bitty so try a few large scale plants. You can also make your garden look wider by widening the borders at the sides and introducing angles or curves that stop the eye going straight to the back fence.
You can also break up the expanse of fence by attaching horizontal wires to vine eyes screwed in at 12 to 18 inch intevals and then using them to train climbing roses, clematis and/or honeysuckle to give flowers and perfume through the groing season.