Preheat the oven to 220ºC/ fan 200ºC/ gas 7. Place the pork in a large roasting tin and brush with the oil. Cook for 30 minutes, then turn down the heat to 190ºC/ fan 170ºC/ gas 5.
Add the potatoes to the pan and cook for 1 hour, 15-30 minutes until the pork is cooked and the potatoes are crisp.
Remove from the oven. Place the pork and potatoes in a serving dish and keep warm.
Meanwhile, cook the carrots in a large pan of boiling water for about 4 minutes. Add the broccoli and cook for a further 3-4 minutes, until the veg are tender. Drain and keep warm.
To make the gravy, pour off all but 2 tablespoons of pork juices left in the roasting pan. Place on the hob, stir in the flour and cook for 1 minute. Remove from the heat and gradually add the beef stock, stirring. Return to the heat and bring to the boil, stirring in the mustard.
Carve the pork and serve with the roast potatoes, vegetables and gravy. Keep the leftover pork to use in sandwiches.