Grate the garlic and ginger and put to one side in a bowl. Chop the chillies as finely as possible and mix them in with the garlic and ginger. Heat a good splash of oil in a pan and add the mustard seeds. When they start to pop, add them to the ginger and garlic mixture along with the paprika, cumin, ground coriander and 2 T of the garam masala. Put half of this mix in a bowl, add the yogurt and the chicken pieces to it, stir and leave to marinate for about a half an hour.