Halve the cucumber lengthways, scrape out the seeds with a spoon and cut into fine slices. Stir the sour cream into the cucumber and season with vinegar, sugar, salt, pepper, cayenne pepper and olive oil.
Peel the pre-cooked potatoes, cut into slices and fold into the sour cream cucumbers together with the chopped parsley. Cut the spring onions into rings and add to the salad.