Preheat the oven to 150°C / gas mark 2.
Unwrap the lamb and place it in a roasting pan. Spoon over one tablespoon of the oil, sprinkle with the rosemary and season with salt and pepper. Roast for three hours.
Place the garlic bulbs around the meat. Drizzle with the remaining oil and continue cooking for a further 20 minutes, until the garlic is tender. Remove the lamb and garlic from the pan, place on a serving plate and cover loosely with foil. Leave to rest in a warm place.
Spoon off as much fat as possible from the roasting pan. Sprinkle over the flour and stir into the pan juices. Gradually add the wine. Place the pan on a gas hob and cook gently for one minute, stirring constantly with a wooden spoon.
Slowly add the stock and redcurrant jam. Bring to the boil and simmer until the jam has melted and the gravy thickened, stirring constantly. Season to taste and strain into a warmed gravy jug.
Serve slices of the hot roast lamb with the gravy, mashed garlic and seasonal vegetables.
Cook's note: The lamb is cooked to medium for this recipe. For well-done meat, increase the initial cooking time by 30 minutes, and cover with foil for the last half an hour. Test the internal temperature with a meat thermometer if unsure.