Lightly coat the chicken in flour, shaking off the excess. Heat the oil in a large non-stick frying pan over a medium heat. Add the garlic and chilli flakes and cook, stirring, for about 1 minute or until the garlic is tender.
Add the chicken to the pan and cook until golden brown, about 4 minutes on each side. Remove the pan from the heat, pour in the brandy, return the pan to the heat, then cook for about 1 minute or until the brandy has evaporated.
Add the grape juice, salt, rosemary and pepper. Bring to a simmer, cover with a lid and cook until the chicken is just done, about 3 minutes.
Add the grapes to the pan and bring to the boil. Push the chicken to one side of the frying pan. Mix together the cornflour and water until you have a paste. Add the mixture to the pan and cook, stirring constantly, until the sauce is slightly thickened, about 1 minute. Serve the chicken with the grape sauce drizzled over the top.