Return the lamb shanks to the pan and pour over the stock and stir through the tomato purée and Worcestershire sauce until combined. Season well and bring to the boil then reduce to a simmer and cook gently, partially covered, for 2 ½ hours, turning the lamb occasionally, until the lamb shanks are tender and falling apart and the sauce reduced.
Meanwhile make the mash, put the potatoes in a pan of cold salted water, bring to the boil and simmer for 15 minutes until tender.
Drain and tip back into the pot and place over a low heat and allow them to steam and dry then add the butter and cream and egg yolk. Using a potato masher, mash the potatoes until smooth and creamy. Stir in the spring onions and season to taste.
When the lamb is tender, shred the meat from the bone into the sauce, stir in the ketchup and transfer to an ovenproof baking dish and top with the champ mash, using a spoon to make a nice topping. Give the topping an extra sprinkle of ground black pepper and bake in the oven for 25 mins until the top is starting to brown and the mince is bubbling up around the edges.