Place the potato pieces in a large pot and fill with cold salted water to just above the level of potatoes.
Boil potatoes 15-20 minutes until soft but still holding their shape. Drain and leave to cool.
In a serving bowl, Mix together the mayonnaise, sour cream, vinegar, mustard, sugar, and salt and pepper to taste.
Add the potatoes, dill pickles, celery and onion and mix to combine.
Leave for at least an hour for the flavours to mingle.
The salad is best served chilled but not too cold – you need the flavours to be vibrant, and coldness can inhibit that. If it’s been stored in the fridge I like to bring it out about half an hour or 45 minutes before eating to let it come to room temperature just a little.