Heat 1 tablespoon of the oil in a heavy-based frying pan. Fry the mustard seeds, curry leaves, ground turmeric and ginger paste, stirring often, for 3-5 minutes, until fragrant.
Add the crab meat, salt and chopped cilantro, mixing well, and cook for a further 2-3 minutes.
Mix together the water and flour to make a flour paste.
Fold a pastry sheet over to make a strip. Place a portion of the crab mixture in one corner of the pastry strip and fold the pastry over into a triangle. Fold the triangle along the strip, sealing the edges with the flour paste.
Repeat the process making 24 samosas in all.
Heat the oil for deep frying in a deep fat-fryer or wok until very hot. Fry the samosas in batches until golden-brown, remove with a slotted spoon and drain on kitchen paper.