Clean crabs and poach them, then drain well. Remove the pincers and crack with a pestle. Trim the legs and cut each crab into four pieces. Set aside.
Heat margarine in a wok until melted then add the milk. Gently simmer the mixture over a medium heat. Add the curry leaves, chilli padi and seasoning. Simmer for 10-15 minutes. Add thickening and stir gently over a very low heat until sauce is thick and glossy in texture.
Return crabs to the mixture and cook over a very low heat for about 1 minute. Dish out the crabs with the curry leaves, chilli padi and a little of the sauce. Serve immediately.