Bring a large pot of water to the boil. Add potatoes and cook until tender, approximately 15 minutes, drain. Set aside to cool in a large salad bowl.
In a small bowl combine Greek yoghurt, wholegrain mustard, Dijon mustard and dill. Stir well to combine.
Add red onion, celery, spring onion and dressing to the potatoes and stir gently to combine.
Refrigerate for at least an hour before serving.