Cut 4 green blue swimmers into quarters with a cleaver or large knife. With a hammer, gently crack the claws and harder sections of shell. Finely chop 6 shallots and 2 stalks lemon grass. Steep 2 teaspoons tamarind in half a cup of boiling water. Chop a handful of fresh coriander leaves. In a food processor, grind together 3 cloves garlic, 1 tablespoon fresh galangal, 2-3 birdseye chillies, seeded, 4 candlenuts (or substitute 8-10 cashews) and 1 teaspoon blachan (hard dark brown shrimp paste), 1 teaspoon turmeric and salt and pepper to taste. Heat 3 tablespoons oil in a large wok or pan, and fry the paste until fragrant. Add shallots, lemon grass, crab pieces and 2 cups coconut milk. Simmer for quarter of an hour. Strain the tamarind water and add half to the sauce. Taste and add more if you wish. Ladle curvy into a serving dish and scatter the fresh coriander on top. Serve with plain rice.