Simmer the potatoes for 15 minutes until tender, then drain, let cool slightly, then quarter.
Meanwhile, put the ingredients for the dressing in a small bowl, season with a little salt & freshly ground pepper, then whisk together.
Put the potatoes into a bowl and whilst still warm pour over half of the dressing, leave to cool
Add the onion, chickpeas, tomatoes, olives & parsley, then the remaining dressing, toss together and serve at room temperature.