Place the chicken in a bowl with the yoghurt & tandoori paste and stir. Marinate for at least 15 minutes, or covered in the overnight in the fridge.
Preheat the grill to high. Take the chicken out of the marinade(reserving the liquid for later) and spread over a baking sheet. grill for 6 minutes each side until lightly charred.
Melt the butter in a casserole dish or large frying pan over a low heat and soften the onions for 10 minutes , stirring occasionally until just golden.
Stir in the garlic, ginger, bay leaves, salt and all the spices.Cook for 1-2 minutes until the spices release their aroma. Stir in the passata, reserved yoghurt marinade and the water.Simmer very gently fro 5 minutes until the sauce thickens until sauce thickens, adding extra water should it get to thick.Stir in the cooked chicken.