How to Make It
Preheat the oven to gas 9 240°C fan 220°C. Using a sharp knife make several 2.5cm deep cuts in the lamb. Rub the olive oil all over the lamb and season generously. Poke a few rosemary sprigs into the slashes. Place the joint in a roasting tin and pour over 400ml of the wine the stock and add the garlic. Cover tightly with foil transfer to the oven and turn the heat down to gas 3 170°C fan 150°C.
Roast for 3 1/2 hours checking occasionally to make sure that all the liquid hasn't evaporated add a little water if it looks too dry. Remove the foil and very carefully drain off the lamb fat from the tin. Add the passata Worcestershire sauce remaining red wine and beans to the roasting tin and return to the oven uncovered for 30 mins. When the lamb is ready remove it from the oven place on a warmed plate cover loosely with foil and allow it to rest for 15-20 mins.
Meanwhile bring a medium pan of water to the boil and cook potatoes for 15 mins or until tender. Drain and mash with the milk and butter. Season well. To serve thickly slice the lamb and serve with the mash and bean mixture. Set aside the leftover meat until cool. Wrap in foil or place in a lidded airtight plastic box and store in the fridge. Use within 2 days.