Pour the oil into a medium pan and set over a medium-high heat. When hot, put in the mustard seeds, urad dal and chillies. As soon as the seeds begin to pop, a matter of seconds, put in the curry leaves and onion. Lower the heat to medium. Stir and fry the onion for about 3 minutes, or until it has softened but not browned. Add the tomato, ground coriander, cayenne pepper and garam masala. Stir for 1 minute. Add the potatoes and stir another minute. Now put in 250 ml/8 fl oz water and the salt. Bring to the boil. Cover, lower the heat and cook for 15-20 minutes, or until the potatoes are tender. Finally, add the coconut milk and fresh coriander. Stir and heat through.