1 3/4 - 2 kg (3 1/2 - 4 lb) roasting chicken, skinned, giblets removed
3 x 141 g (5 oz) cartons natural unsweetened yogurt
1 garlic clove, crushed with 1/2 tsp salt
2 pieces fresh root ginger, finely chopped, or 1 tsp ground ginger
2 dried chilis, crushed, or 1/2 tsp chili powder
1/2 tsp ground turmeric
1/2 tsp ground mixed spice
2 tbsp freshly chopped mint
1 tbsp ground cumin seeds
1 tsp salt
2 tsp sugar
1 bunch of watercress to garnish