Tip the potatoes into a large saucepan of salted water and bring to the boil. Cook for 10-15 minutes, depending on their size, or until tender. Drain and leave to cool.
Meanwhile, mix together the Cr*me Fraiche, mustard, vinegar and dill then season with salt and pepper to taste.
Mix the dressing with the cooled potatoes until well coated, then toss in the watercress.