Boil the potatoes until they are done, let them cool and peel them. Slice.
Peel the cucumber, cut in half lengthways and completely scrape out the stones. Halve the halves lengthways again, then cut into slices and add to the potatoes.
In an extra bowl, mix the remaining ingredients into a light dressing. Pour everything over the salad, stir carefully and leave it to stand in the refrigerator overnight.