To make the dressing, put the tamarind pulp in a small pan, pour over 75ml boiling water and add the sugar, cumin and ginger. Simmer, without a lid, until the dressing thickens and becomes syrupy, about 10-15 mins. It should have a pleasant tang - add extra sugar if needed.
While the tamarind is cooking, bring a large pan of salted water to the boil and add the potatoes. Return to the boil and cook for 15 mins until tender. Drain, then cool slightly before halving them. Transfer to a big mixing bowl and spoon over the dressing, gently stirring to make sure the potatoes are well coated. Drizzle over the yogurt, scatter over the coriander, and serve warm or at room temperature.