Mix together in a large bowl the vinegar, mustard, garlic, lemon juice, dark brown soft sugar, salt and pepper.Whisk in the olive oil before adding the chicken to the marinade. Cover and marinate in a fridge for 8 hours or overnight.Preheat BBQ and add the chicken breasts.Carefully cook on both sides. Don't burn too much as they dry out but they should have a little black on them. Add marinate while cooking to keep moist.Optionally boil the left over marinate for about 10 minutes and use it as a sauce.Serve with whatever you want - goes well with fried potatoes.