Spray a wide non-stick frying pan with low calorie cooking spray and place over a high heat. Add the chicken, peppers and onion and stir-fry for 5 minutes or until lightly browned.
Add the diet cola, stock, passata, tomato purée, garlic, Worcestershire sauce, soy sauce and dried mixed herbs and stir well. Bring to the boil, cover, reduce the heat to medium-low and simmer for 12-15 minutes.
Stir in the sugar snap peas and turn the heat to medium-high. Cook for another 10-15 minutes or until the chicken is cooked through and the veg is tender.