Preheat the oven to 160°C.
Heat the oil in a large ovenproof casserole.
Add the lamb and fry in batches, over a high heat until browned.
Remove with a slotted spoon and set aside.
Add the butter and when foaming add onions, leeks and carrots to the pan and cook for 3 - 4 minutes, stirring occasionally.
Return the lamb to the pan and pour the stock and Worcestershire sauce over the casserole.
Add thyme and rosemary sprigs and season with salt and pepper.
Remove from the heat, arrange the potatoes over the top and brush them with the melted butter.
Season again, cover and bake in the oven for 1½ hours, then remove the lid and cook for a further 30 - 45 minutes or until the meat and potatoes are tender.