In a bowl, stir together yogurt, lime juice, curry powder, cumin, salt, cayenne, sumac and 1 tbsp. garlic. Transfer 3/4 cup spiced yogurt to a small container and chill until ready to use. Stir remaining 1/2 tbsp. garlic into yogurt mixture in bowl. Add lamb pieces and stir to coat evenly. Cover and chill at least 8 hours.  Preheat a grill pan to medium-high, then oil well with a piece of oiled paper towel.  Remove lamb pieces from marinade (discard marinade) and thread onto 6-8 skewers or kebab sticks.  Grill, uncovered, until charred outside and just cooked through (cut to test), about 5 minutes per side. Remove kebabs from the pan, oil the pan again and grill the pita bread until well charred. Leave them to absorb the oily lamb flavours in the pan. Sprinkle skewers with mint and serve with pita, and reserved yogurt mixture (thinned with a little milk or water if you like).