To calculate the cooking time for the lamb, add 450g (1lb) to the pack weight. Allow 25 minutes per 450g for well done, 20 for medium and 15 for rare. Slash the lamb skin and place in a large non-metallic dish. Blend the marinade ingredients in a food processor until finely chopped. Pour over the lamb, working well into the cuts. Cover and marinate for several hours. Reserve the marinade. Roast the joint in the centre of a preheated oven at 190?C, 375F, gas mark 5, until there is 1 hour of the cooking time left. Add the vegetables. Season well, brush with marinade and drizzle with a little olive oil. Cook for the remaining hour turning the vegetables occasionally. Remove the lamb and vegetables from the roasting tin. To make the gravy, stir the flour into the remaining marinade and make up to 425ml ( 3/4 pint) with water. Skim the fat from the cooking juices and add the marinade. Cook until the gravy thickens. Season and serve with the lamb and vegetables.