Put the ginger, garlic, 1/4 tsp salt and the lemon juice into a blender. Add about 2 tbsps water and blend until smooth.
Place the chicken pieces in a non metallic bowl, pour the ginger mixture over the top and rub in. Cover the bowl with cling film and refrigerate for at least 30 minutes and up to 24 hours.
Put the tomatoes, coriander, chillies, tomato puree, 3/4 tsp salt and 2 tbsps water into a blender and blend until smooth.
Pour the oil into a large non stick lidded pan and set over a high heat. When very hot put in the chicken pieces together with the marinade. Fry, stirring, for about 10 minutes until the chicken pieces are light brown.
Add the tomato mixture from the blender and continue to cook, stirring, until the sauce is thick and clings to the chicken and the oil separates from it, about 10 minutes.
Add the yoghurt and stir and cook for 4-5 minutes until the yoghurt disappears and leaves a thick sauce edged with oil. Cover, reduce the heat as low as possible and cook for 5-10 minutes more until the chicken is tender. Stir in 1-2 tbsps water if you think it is drying out too much.