Pre heat the oven to 180ºC, 160ºC fan, gas 4. Mix together the ketchup, mustard, vinegar, chilli and sugar. Pour into a freezer bag.
Make deep slashes in the surface of each chicken breast, this will improve the overall flavour of the finished chicken.
Coat the chicken with the glaze and place in a roasting tin and roast for 20-30 minutes. Pop under a preheated grill for 2 minutes to sear the chicken a little if you wish.
Serve with a leaf salad and a summer potato salad.
BBQ Tip: to finish on the BBQ: Sear the cooked chicken on the BBQ for 5 minutes to give that smoky flavour.