Add all of the sauce ingredients to a food processor and blitz together. Add a little oil at a time to get the consistency you want. The sauce should be thick enough but able to pour easily.
Now for the chicken
Place a large pan (with tight fitting lid) over a medium high heat. Add the butter and vegetable oil and bring up to heat.
Add the chopped onions and a pinch of sale and cook for 15 minutes until lovely and soft and slightly golden.
Add the tikka paste you just made, or add shop brought sauce to the pan with the peppers and cook for about another 5 minutes.
Add the chicken cubes stir thoroughly to coat with all of the paste. Cook for a further 3 minutes then add the chopped tomatoes, tomato puree and the water. Place the lid on and simmer for at least 15 minutes until the chicken is full cooked.
Add the mango chutney, natural yoghurt and cream and stir everything together.
Allow the contents to warm through gently before serving.