Heat the oil in a heavy 3 1/2 l pot over a medium high flame. When hot, put in the finely chopped garlic. Stir for about 5 seconds. Now put in the red pepper and cumin seeds. Stir for another 3 second. The garlic should brown lightly, the red pepper should darken, and the cumin seeds should sizzle. Lower the heat to medium, put in the grated coconut and stir it around for 10 to 15 seconds.
Drain the potatoes. Add them as well as the turmeric, ground cumin, tomatoes (including any juice that may have accumulated or the juice in the tin), the salt, and 3 1/2 dl of water. Bring to the boil. Cover, turn heat low, and simmer for about 45 minutes or until potatoes are tender. Stir gently every 7 to 8 minutes during this cooking period.
Put in the sugar and vinegar. Stir again and cook, uncovered, for 1 minute.