Preheat the oven to 180°C / 350°F / Gas mark 4.
Use a heat proof dish, large enough to fit the lamb and scatter over all the vegetables, fitting them snugly along with the herbs.
Roughly stir in the tomato purée.
Sit the lamb on top, pour over the wine and season with salt and pepper generously.
Cover the dish or wrap foil over and cook undisturbed for 3 hours.
Remove from the oven, leave to cool, then chill overnight.
The next day, remove all the hard fat from around the lamb and discard.
Lift the lamb out of the dish, removing away and keeping any of the jellylike juices.
Pull apart the meat, discarding any large pieces of fat and the bones.
Put the lamb back in the dish, then spoon over the jellied sauce and mix well with the vegetables and set aside.
Heat oven to 200°C / 400°F / Gas mark 6.
To make the topping, peel and slice the potatoes into rounds.
Place in a pan of cold, salted water, bring to a hard boil and drain.
Tip back into the pan, pour over the cream and season with salt and pepper.
Pile the potatoes over the lamb, drizzle over any remaining cream from the pan and dot with butter.
Bake for 40 mins-1 hour until the top is golden and the sauce is just starting to bubble.