In a small bowl, mix all of the marinade ingredients together.
Place the chicken breasts on a board, cover with cling film and bash with a rolling pin to thin out slightly. Place the chicken breasts in a bowl and pour over the marinade, turning them so they are completely covered. Cover the bowl with cling film and place in the fridge for at least 4 hours.
Once the chicken has marinated, preheat the oven to 120 oC / 250 oF. Place the marinated chicken breasts on to a roasting tin & spoon over any remaining marinade. Cook on the middle shelf of the oven for 2 hours. Note, that at this temperature the chicken will not be thoroughly cooked at this stage (so no picking!).
Heat the oil in a large pan and add the chopped onions. Cook for 10 minutes, stirring occasionally to make sure that they do not stick to the pan. If they look like they might, add a splash of water.
Add the grated ginger and garlic and a splash of water,stir, then add the paprika, turmeric and ground coriander. Stir, then cook for 1 minute.
Add the tomato puree and stir for 1 minute before adding the tinned tomatoes and the water. Bring this mixture to the boil, then reduce the heat and simmer for 5 - 10 minutes.
Add the chopped peppers and cook for 5 minutes. Remove the pan from the heat and set aside.
Once the chicken is ready to be removed from the oven, take it out and if you have one, use a cooks blow torch to blacken bits of the marinaded chicken. If you do not have a blow torch, don't worry, you can skip this part. Set aside for 10 minutes before cutting up into bite sized chunks.
Place the curry sauce back on a medium heat and heat through. Add the chicken pieces and simmer for 5 - 10 minutes until the chicken is cooked through.
Stir in the yogurt, season to taste and sprinkle with chopped coriander.
Serve with rice and/or garlic naan