Heat a large frying pan over a medium-high heat for 1-2 minutes. Whilst the pan is heating up, brush the chicken breasts with the olive oil and season well. Reduce the heat a little and pan-fry the breasts for 15-17 minutes, turning occasionally until they are golden. Cut into a thick chunk and check that it is cooked through, with no pink showing. As the breasts cook, create the barbecue sauce by heating the sunflower oil in a medium sauce pan over a medium heat. Fry the garlic for a minute, stirring constantly, then add the sugar, soy sauce and tomato ketchup. Mix well, bring to the boil then simmer for 5 minutes. Remove from the heat and set to one side. When the chicken breasts are cooked; remove from the pan and allow to rest for a few minutes before shredding into a mixing bowl using two forks. Marinade in the barbecue sauce, making sure you reserve some of the sauce to serve in a small serving pot alongside the sandwiches. Create the sandwiches by spooning the marinaded chicken onto the bottom half of the rolls. Top with a lettuce leaf and then the top half of the rolls to create sandwiches. Serve alongside a pot of the barbecue sauce.