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Put the potatoes in a saucepan of cold, salted water and bring to the boil. Simmer for 12-15 minutes, or until tender. Drain well, then leave to cool completely in a colander so they steam dry.
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Meanwhile, mix the vinegar and mustard powder to a paste in a large mixing bowl, then stir in the soured cream, followed by the salad cream to make a smooth sauce. Stir in the mustard seeds and grainy mustard, followed by the chopped shallot and celery. Season with salt and white pepper, and keep in the fridge until the spuds have cooled.
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Mix the cooled spuds into the mustard dressing and chill until a good half an hour before serving, when you want to bring them back to room temperature. Stir in most of the cress, then scatter the rest on top.
Rashmi Sharma 
