Preheat the oven to 190ºC/ fan 170ºC/ gas 5. Put the chicken in a large roasting tin and brush with the oil. Cook for 30 minutes, then add the potatoes. Cook for a further 50 minutes, until the chicken is cooked through and the potatoes are golden brown and crisp. The chicken will be cooked through when the juices run clear.
Transfer the chicken and potatoes to a serving dish and keep warm.
Meanwhile, cook the carrots in a large pan of boiling water for about 8 minutes until almost tender, adding the peas and broccoli for the final 3-4 minutes.
To make the gravy, pour off all but 2 tablespoons of chicken juices from the roasting pan. Stir the flour into the remaining chicken juices and cook on the hob for 1 minute. Remove from the heat and gradually add the stock. Return to the heat and bring to the boil, stirring. Serve the chicken with the roast potatoes, mixed vegetables and gravy.