My thought is to bring in lots of top soil and compost - say to a depth of 20-30cm where you want to plant.

Testing the pH might save you planting acid loving plants where the soil is on the alkaline side, but it sounds as though you need more depth of soil. Have you thought of making raised beds?

Thanks Kildermorie and Steragram.
We are putting lots of compost, mixed with what soil we have into some raised beds, and planting anything we know grows in this area in the ground (mainly large trees and shrubs which we have seen in our friends' gardens. Very hard digging holes for them as we have to use a mixture of fork, spade and a metal farmer's hoe. And loads of effort and elbow grease. Back breaking work, but we will get there.

Welcome to GOY. What part of Spain do you live, would like to see how your garden progressing, when I have been to Spain I have noticed Garden Centre's, do you ever visit them.
Do write a profile about yourself and add some pictures, it's nice to know how one's garden is doing :o))

Penelopedot, Hope you are well rewarded for all your hard work. We would enjoy seeing some before and after photos.