Bring a large pan of salted water to the boil then add the potatoes. Return to the boil and cook for 15 minutes. The potatoes should be just tender to the point of a knife. Drain in a colander and cool for 5-10 minutes.
While the potatoes are cooking, trim the roots from the spring onions then slice the white bulb ends. Reserve the green stems. Heat the oil in a deep saucepan, add the mustard seeds and cook until they start to jump and pop. Add the chopped spring onion and curry paste. Cook, stirring continuously, for 2 minutes.
Tip the mixture into a large bowl then stir in the yogurt and mayonnaise with plenty of salt and ground black pepper. Now you have a choice: for everyday you could the keep skins on salad potatoes, but for entertaining it's 10 minutes well spent to remove them – they look better and take on the flavour of the dressing more easily.
Chop all but 2 of the green spring onion stems and add to the dressing along with the potatoes and celery and carefully fold everything together until evenly coated. Reserve the remaining stems in water. Cover the bowl with cling film and chill until ready to eat.
To serve remove the potatoes from the fridge a good hour before serving so they return to room temperature. If the consistency of the dressing looks a little thick, thin it with a splash of milk or water. Pile the salad into a serving bowl. Cut the reserved spring onion stems into long shreds and scatter them over the salad along with a few mustard seeds.