Heat the sunflower oil in a large stockpot, over medium heat. Stir in the onions and saute 5 minutes. Stir in the garlic, ginger, curry paste and garam masala and continue to cook, stirring frequently for 3 minutes.Fold in the potatoes and chickpeas and stir to coat in spices. Add the tomatoes and coconut milk, and bring to a simmer. Season generously with salt and pepper, then lower the heat and partially cover. Let simmer, stirring occasionally, until the potatoes are tender, 15-20 minutes, adding a little water if the sauce evaporates too much.