Put the potatoes in a large pan of slightly salted water and bring to the boil. Turn down the heat and gently simmer for 10-15 minutes until they are tender. Drain, then let them cool.
Meanwhile, crumble the feta into a small bowl and mix with the oil and yogurt.
Cut the onion into tiny pieces and add to the mixture together with the lemon zest and garlic, if used. Season to taste.
When cooled, cut the potatoes into bite-sized pieces.
Put them in a large mixing bowl and pour over the feta mixture. Add the dill and combine well.
Add the olives just before serving.