Scrub and clean the potatoes, then put them in a pan of lightly salted water. Bring to the boil and simmer for 10-15 minutes until tender.
Meanwhile, mix the vinegar, oil and mustard. Add a pinch of salt and ground pepper.
Drain the potatoes and put them in a serving bowl. Pour over the vinegar mixture and leave them to cool
When the potatoes have completely cooled, add the yogurt & mayonnaise then combine gently.
Mix in the spring onions and season to taste just before serving. (You can add some chopped herbs of your choice too, if you like).