Pound the chillies and garlic in a mortar with a pestle, or process in a blender until you have a rough paste. Gradually add the oil, and blend until it has a smooth consistency.
Put the chilli purée into a heavy-based pot over low heat with the sea salt and sugar and cook, stirring occasionally. Cook for about 45 minutes, or until the sauce turns bright red and the rawness has been cooked out. Remove from the heat and stir in the soy sauce. Adjust the seasoning to taste.