Preheat your oven to 160C.
Mix together the rapeseed oil, garlic, salt, thyme, cumin and paprika. Melt the butter separately, and pour into the spices, mixing well.
Place the lamb in a casserole dish, and coat well with the spice mix, rubbing in to cover the entire surface. Put the casserole dish in the oven, without a lid, and cook for 35 minutes.
After 35 minutes, remove from the oven, and pour in the boiling water - be sure to pour around the lamb, so you don't end up washing the spices off the top. Gently baste the lamb with this liquid, and return to the oven.
Cook for a further three hours, basting the lamb every hour. Add more water if it seems to be getting dry.
Serve! You can also make a gravy with the juices, should you wish - be warned, though, that it'll be pretty rich, and you may want to add more liquid before thickening to dilute the flavour.
